Magnetoelectric multilayer composites for field conversion

ABSTRACT

Magnetoelectric multilayer composites comprising alternate layers of a bimetal ferrite wherein one of the metals is zinc and a piezoelectric material for facilitating conversion of an electric field into a magnetic field or vice versa. The preferred composites include cobalt, nickel, or lithium zinc ferrite and PZT films which are arranged in a bilayer or in alternating layers, laminated, and sintered at high temperature. The composites are useful in sensors for detection of magnetic fields ( 10 ); sensors for measuring rotation speed, linear speed, or acceleration; read-heads in storage devices by converting bits in magnetic storage devices to electrical signals; magnetoelectric media for storing information; and high frequency devices for electric field control of magnetic devices or magnetic field control of electric devices.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. application Ser. No.10/125,670, filed Apr. 18, 2002, now U.S. Pat. No. 6,835,463.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was supported by National Science Foundation Grant No.DMR-0072144. The U.S. government has certain rights in this invention.

REFERENCE TO A “COMPUTER LISTING APPENDIX SUBMITTED ON A COMPACT DISC”

Not Applicable.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to magnetoelectric multilayer compositescomprising alternate layers of a magnetostrictive material, which is abimetal ferrite wherein one of the metals is zinc, and a piezoelectricmaterial such as lead zirconate titanate (PZT), lead zincate niobate(PZN), lead zincate niobate lead-titanate (PZN-PT), lead magnesiumniobate lead-titanate (PMN-PT), lead lanthanum zirconate titanate(PLZT), Nb/Ta doped-PLZT, and barium zirconate titanate (BZT) forfacilitating the conversion of an electric field into a magnetic fieldor vice versa. The preferred composites include cobalt, nickel, orlithium zinc ferrite and PZT films which are arranged in a bilayer or inalternating layers, laminated, and sintered at high temperature. Thecomposites are useful in sensors for detection of magnetic fields;sensors for measuring rotation speed, linear speed, or acceleration;read-heads in storage devices by converting bits in magnetic storagedevices to electrical signals; magnetoelectric media for storinginformation; and high frequency devices for electric field control ofmagnetic devices or magnetic field control of electric devices.

(2) Description of Related Art

Conversion of electric to magnetic fields and vice versa plays animportant role in many devices. One way in principle to accomplish thisis a composite of magnetostrictive and piezoelectric materials. In suchcomposites, the field conversion is a two step process: magnetostrictioninduced mechanical deformation resulting in induced electric fields.Until now, interest in such transducers has been lacking because ofconversion efficiencies that are an order of magnitude below theoreticalpredictions.

The magnetoelectric (ME) effect is defined as the dielectricpolarization of a material in an applied magnetic field or an inducedmagnetization in an external electric field (Landau and Lifshitz,Electrodynamics of Continuous Media (Pergamon, Oxford, 1960), p. 119).In materials that are magnetoelectric (ME), the induced polarization Pis related to the magnetic field H by the expression, P=αH, where α isthe second rank ME-susceptibility tensor. A parameter of importance isthe ME voltage coefficient α_(E)=δE/δH with α=ε₀ε_(r)α_(E) where ε_(r)is the relative permittivity. The effect, first observed inantiferromagnetic Cr₂O₃, is generally weak in single phase compounds(Astrov, Soviet Phys. JETP 13: 729 (1961); Rado and Folen, Phys. Rev.Lett. 7: 310 (1961); Foner and Hanabusa, J. Appl. Phys. 34: 1246 (1963);Tsujino and Kohn, Solid State Commun. 83: 639 (1992); Bichurin,Ferroelectrics 204: 356 (1997); Kornev et al., Phys. Rev. B 62: 12247(2000)). A strong ME effect, however, could be realized in a“product-property” composite consisting of magnetostrictive (MS) andpiezoelectric (PE) phases in which the mechanical deformation due tomagnetostriction results in a dielectric polarization due topiezoelectric effects (Suchtelen, Philips Res. Rep. 27: 28 (1972)). Vanden Boomgaard synthesized bulk composites of cobalt ferrite or nickelferrite with BaTiO₃ that yielded ME voltage coefficient α_(E) that was afactor 40–60 smaller than calculated values (Van den Boomgaard et al.,J. Mater. Sci. 9: 1705 (1974); Van den Boomgaard et al., Ferroelectrics14: 727 (1976)). Possible causes for such low α_(E) include microcracksdue to thermal expansion mismatch between the two phases, leakagecurrent through low resistivity ferrites, porosity, and any impurity orundesired phases.

A multilayer structure is expected to be far superior to bulk compositessince the PE layer can easily be poled electrically to enhance thepiezoelectricity and the ME effect. In addition, MS layers enclosed inmetal electrodes lead to series electrical connectivity for PE layersand further enhancement of piezoelectricity (Harshe et al., Int. J.Appl. Electromagn. Mater. 4: 145 (1993); Avellaneda and Harshe, J.Intell. Mater. Syst. Struct. 5: 501 (1994)). Harshe and co-workersproposed a theoretical model for a magnetostrictive-piezoelectricbilayer structure. The estimated α_(E) for cobalt ferrite (CFO)-leadzirconate titanate (PZT), or -barium zirconate titanate (BZT), bilayerwas in the range 0.2–5 V/cm Oe, depending on field orientations,boundary conditions, and material parameters (Harshe, Ph.D. thesis,Pennsylvania State University (1991)). They also prepared multilayers bysintering tape-cast ribbons (Harshe et al., Int. J. Appl. Electromagn.Mater. 4: 145 (1993); Avellaneda and Harshe, J. Intell. Mater. Syst.Struct. 5: 501 (1994)). Samples of CFO—BaTiO₃ structures did not show MEeffects. The largest α_(E)=75 mV/cm Oe, a factor of 3–30 times smallerthan theoretical values, was measured for CFO-PZT. The low α_(E) or theabsence of ME effect is most likely due to unfavorable interfaceconditions and the following problems due to the use of platinumelectrodes at the interface: (i) the electrode makes it a three-phasemultilayer structure and leads to poor mechanical coupling between thetwo oxide layers, (ii) platinum with thermal expansion coefficient muchhigher than that of oxides will result in micro-cracks at the interfaceduring sample processing, (iii) measurement conditions for ME effectsmight correspond to the inelastic region of stress-straincharacteristics for Pt leading to a reduction in ME coefficients.

In summary the use of appropriate MS and PE phases and the eliminationof foreign electrodes are critically important for obtaining large MEeffects in the multilayer (ML) structures. However, current materialsavailable for making magnetostrictive and piezoelectric compositesproduce composites which have conversion efficiencies that are an orderof magnitude below theoretical predictions. The present inventionprovides a novel class of materials for making magnetostrictive andpiezoelectric composites that have a large ME effect and maximum fieldconversion efficiency.

SUMMARY OF THE INVENTION

The present invention provides magnetoelectric multilayer compositescomprising alternate layers of a magnetostrictive material, which is asintered bimetal ferrite wherein one of the metals is zinc, and apiezoelectric layer such as lead zirconate titanate (PZT), lead zincateniobate (PZN), lead zincate niobate lead-titanate (PZN-PT), leadmagnesium niobate lead-titanate (PMN-PT), lead lanthanum zirconatetitanate (PLZT), Nb/Ta doped-PLZT, and barium zirconate titanate (BZT)for facilitating the conversion of an electric field into a magneticfield or vice versa. The preferred composites include cobalt, nickel, orlithium zinc ferrite and PZT films which are arranged in a bilayer or inalternating layers, laminated, and sintered at high temperature toproduce the composite. The composites are useful in smart sensors fordetection of magnetic fields; sensors for measuring rotation speed,linear speed, or acceleration; read-heads in storage devices byconverting bits in magnetic storage devices to electrical signals;magnetoelectric media for storing information; and high frequencydevices for electric field control of magnetic devices or magnetic fieldcontrol of electric devices.

Therefore, the present invention provides in a magnetoelectric compositecomprising at least one piezoelectric composition and at least onemagnetostrictive composition as separate layers joined together, theimprovement which comprises a sintered bimetal ferrite as themagnetostrictive composition, wherein one of the metals is zinc, andwherein the composite has a magnetoelectric voltage coefficient of atleast 100 mV/cm Oe.

In a particular embodiment of the magnetoelectric composite, thepiezoelectric material is selected from the group consisting of leadzirconate titanate (PZT), lead zincate niobate (PZN), lead zincateniobate lead-titanate (PZN-PT), lead magnesium niobate lead-titanate(PMN-PT), lead lanthanum zirconate titanate (PLZT), Nb/Ta doped-PLZT,and barium zirconate titanate (BZT).

In a preferred embodiment of the magnetoelectric composite, the bimetalferrite has the formula:Co_(1−x)Zn_(x)Fe₂O₄where x is 0.2 to 0.5. Alternatively, the bimetal ferrite is a nickelzinc ferrite which has the formula:Ni_(1−x)Zn_(x)Fe₂O₄where x is 0.1 to 0.5 or a lithium zinc ferrite which has the formula:Li_(0.5−x/2)Zn_(x)Fe_(2.5−x/2)O₄where x is 0.1 to 0.4.

The present invention further provides in a method for forming amagnetoelectric composite wherein a combination of a piezoelectriccomposition and a magnetostrictive composition are joined together inseparate layers, the improvement which comprises (a) providing a bimetalferrite powder, wherein zinc is one of the metals, as themagnetostrictive composition; (b) forming a combination of the powder ofstep (a) with a powder of a piezoelectric material in the separatelayers; and (c) compressing and sintering the combination of step (b) toform the magnetoelectric composite.

In a particular embodiment of the method, the piezoelectric material isselected from the group consisting of lead zirconate titanate (PZT),lead zincate niobate (PZN), lead zincate niobate lead-titanate (PZN-PT),lead magnesium niobate lead-titanate (PMN-PT), lead lanthanum zirconatetitanate (PLZT), Nb/Ta doped-PLZT, and barium zirconate titanate (BZT).

In a preferred embodiment of the method, the bi-metal ferrite has theformula:Co_(1−x)Zn_(x)Fe₂O₄where x is 02. to 0.5. Alternatively, the bimetal ferrite is a nickelzinc ferrite which has the formula:Ni_(1−x)Zn_(x)Fe₂O₄where x is 0.1 to 0.5 or a lithium zinc ferrite which has the formula:Li_(0.5−x/2)Zn_(x)Fe_(2.5−x/2)O₄where x is 0.1 to 0.4.

OBJECTS

It is an object of the present invention to provide materials for makingmagnetostrictive and piezoelectric composites that have a large MEeffect and maximum field conversion efficiency.

Is a further object of the present invention to provide magnetostrictiveand piezoelectric composites that have a large ME effect and maximumfield conversion efficiency and which are useful inter alia formagnetoelectric memory devices, electrically controlled magneticdevices, magnetically controlled piezoelectric devices, and smartsensors.

These and other objects of the present invention will becomeincreasingly apparent with reference to the following drawings andpreferred embodiments.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the magnetoelectric (ME) voltage coefficient α_(E)=δE/δHversus bias magnetic field H for a multilayer sample ofCo_(1−x)Zn_(x)Fe₂O₄ (CZFO) (x=0.4)-lead zirconate titanate (PZT). Thesample contained 11 layers of CZFO and 10 layers of PZT with a thicknessof 18 μm. The data at room temperature and 100 Hz are for transverse(out-of-plane δE perpendicular to in-plane δH (diamonds)) andlongitudinal (out-of-plane δE and δH (circles)) field orientations.There is a 180 degree phase difference between voltages for +H and −H.

FIG. 2 shows the transverse ME voltage coefficient α_(E,31) as afunction of H at room temperature and 100 Hz for multilayers of CZFO-PZTwith x=0.0 (circles), x=0.2 (triangles), and x=0.4 (diamonds).

FIG. 3 shows the variation of peak transverse (filled circles) andlongitudinal (open circles) ME voltage coefficients with zincconcentration x in layered CZFO-PZT. The data at room temperature and100 Hz were obtained from profiles as in FIGS. 1 and 2.

FIG. 4A shows the transverse ME voltage coefficients versus H data as inFIG. 1, but for multilayer samples of Ni_(1−x)ZnxFe₂O₄ (NZFO)-PZT withx=0.0 (triangles), x=0.2 (diamonds), and x=0.4 (circles).

FIG. 4B shows the longitudinal ME voltage coefficients versus H data asin FIG. 1, but for multilayer samples of Ni_(1−x)ZnxFe₂O₄ (NZFO)-PZTwith x=0.0 (triangles), x=0.2 (diamonds), and x=0.4 (circles).

FIG. 5 shows the zinc concentration dependence of peak transverse(circles) and longitudinal (diamonds) ME coefficients in NZFO-PZTlayered samples. The Figure contains the same data as FIGS. 4A and 4B.The line is guide to the eye.

FIG. 6A shows the transverse voltage coefficient versus H profiles as inFIGS. 1 and 4, but for multilayer samples ofLi_(0.5−x/2)Zn_(x)Fe_(2.5−x/2)O₄ (LZFO)-PZT with x=0.0 (circles), x=0.2(squares), and x=0.3 (triangles).

FIG. 6B shows the peak value of α_(E,31) vs x of theLi_(0.5−x/2)Zn_(x)Fe_(2.5−x/2)O₄ (LZFO)-PZT samples of FIG. 6A.

FIG. 7A shows the magnetostriction measured parallel to the appliedfield versus H for CZFO bulk samples made from thick films with x=0.0(circles) and x=0.2 (diamonds). The data are at room temperature and forH parallel to the sample plane.

FIG. 7B shows the magnetostriction measured parallel to the appliedfield versus H for NZFO bulk samples made from thick films with x=0.0(circles), x=0.2 (diamonds), and x=0.5 (triangles). The data are at roomtemperature and for H parallel to the sample plane.

FIG. 7C shows the magnetostriction measured parallel to the appliedfield versus H for LZFO bulk samples made from thick films with x=0.0(triangles), x=0.2 (diamonds), and x=0.3 (circles). The data are at roomtemperature and for H parallel to the sample plane.

FIG. 7D shows λ₁₁ vs H for CZFO(x=0.4)-PZT.

FIG. 8A shows a comparison of theoretical (circles) and measured values(triangles) of the transverse ME voltage coefficient α_(E,31) forlayered samples of CZFO-PZT wherein x=0.0.

FIG. 8B shows a comparison of theoretical (circles) and measured values(triangles) of the transverse ME voltage coefficient α_(E,31) forlayered samples of CZFO-PZT wherein x=0.2.

FIG. 8C shows a comparison of theoretical (circles) and measured values(triangles) of the transverse ME voltage coefficient α_(E,31) forlayered samples of CZFO-PZT wherein x=0.4.

FIG. 9A shows the theoretical (triangles) and measured α_(E,31) versus Hfor NFO(x=0.0)-PZT multilayer composites. The circles are increasing Hand the squares are decreasing H.

FIG. 9B shows the theoretical and measured α_(E,31) versus H forLZFO-PZT multilayer composites with x=0.3 data (filled squares), x=0.3theory (open squares), x=0.0 data (open circles), and x=0.0 theory(filled circles).

FIG. 10 shows a composition dependence of α_(E,31) and initialpermeability (from Toltinnikov and Davydov, Soviet Phys. Solid State 6:1730 (1965)) for CZFO-PZT multilayers with α_(E,31) (circles) and μ_(i)(triangles).

FIG. 11A shows the transverse magnetoelectric (ME) voltage coefficientverses static magnetic field (Oe) for nickel zinc ferrite-PZT (NZFO-PZT)multilayers for zinc concentrations of x=0.0 (filled circles), x=0.1(triangles), and x=0.2 (diamonds). The Figure contains the same data asFIGS. 4A and 4B.

FIG. 11B shows the variation of the peak voltage coefficient with x forthe NZFO-PZT multilayers of FIG. 11A. The Figure contains the same dataas FIGS. 4A and 4B.

FIG. 12 shows parallel (λ₁₁) and perpendicular (λ₁₃) magnetostrictionverses static magnetic field (Oe) for NZFO-PZT multilayers with x=0.0(filled circles), x=0.2 (open circles), and x=0.5 (triangles).

FIG. 13 shows the effect of volume ratio (R) wherein R=volume offerrite/volume of PZT on the transverse voltage coefficient in nickelferrite (x=0.1)-PZT multilayers with R=3.0 (diamonds), R=1.8(triangles), and R=0.4 (circles).

FIG. 14 shows parallel magnetostriction verses volume ratio (R) forNZFO(x=0.1)-PZT multilayers with R=0.3 (filled inverted triangles),R=1.0 (filled circles), R=3.0 (open circles), and bulk (open triangles).

FIG. 15A shows the effect of ferrite layers (n) on the magnetoelectric(ME) voltage coefficient in NZFO(x=0.3)-PZT multilayers. The samplescontained n bimetal ferrite and n−1 PZT layers with n=5 (filled invertedtriangles), n=15 (filled circles), n=25 (open circles), and n=30 (opentriangles).

FIG. 15B shows the transverse (filled inverted triangles) andlongitudinal (filled circles) ME voltage coefficients forNZFO(x=0.3)-PZT multilayers with n bimetal ferrite and n−1 PZT layers.

FIG. 16A shows parallel magnetostriction verses static magnetic field(Oe) NZFO(x=0.3)-PZT multilayers wherein n is the number of NZFO layersand n−1 is the number of PZT layers. The samples contained n ferrite andn−1 PZT layers with n=5 (circles), n=15 (squares), and n=30 (diamonds).

FIG. 16B shows perpendicular magnetostriction verses static magneticfield (Oe) for NZFO(x=0.3)-PZT multilayers wherein n is the number ofNZFO layers and n−1 is the number of PZT layers. The samples contained nferrite and n−1 PZT layers with n=5 (circles), n=15 (squares), and n=30(diamonds).

FIG. 17 shows the bias magnetic field H dependence of themagnetostriction for a multilayer composite of nickel ferrite (NFO) andlead zirconate titanate (PZT). The sample contained n+1 layers of NFOand n layers of PZT, with n=14 and a layer thickness of 14 μm. The dataat room temperature are for H parallel (λ₁₁) (circles) and perpendicular(λ₁₃) (triangles) to the plane of the multilayer composite, and are usedfor theoretical estimates of magnetoelectric-voltage coefficient. λ₁₁ isnegative, but the Figure shows the magnitude.

FIG. 18 shows the transverse magnetoelectric (ME) voltage coefficientα_(E,31)=δE₃/δH₁ at room temperature as a function of static magneticfield H for a two-layer structure consisting of 200-μm films of NFO andPZT. The field H and the 1-kHz ac magnetic field δH₁ are appliedparallel to each other and parallel to the sample plane and the inducedelectric field δE₃ is measured perpendicular to the sample plane. Theopen (filled) circles are data points for increasing (decreasing) H. Thelines are guides to the eye.

FIG. 19 shows the static-field dependence of room-temperature transverseand longitudinal ME voltage coefficients, α_(E,31) and α_(E,33),respectively, for a multilayer composite with n=14. The thickness ofeach layer is 14 μm. For longitudinal ME effects, the fields H, δH (1kHz), and δE are parallel to each other and perpendicular to the sampleplane. The open (filled) circles are data points for increasing(decreasing) H. The lines are guides to the eye.

FIG. 20A shows the frequency dependence of the transverse coefficientα_(E,31) for the multilayer with n=14 at various temperatures (T=degreeskelvin (K)). The lines are guides to the eye. Values of α_(E,31) are forthe bias field H corresponding to maximum value in the ME effect. TheFigure shows the frequency dependence at T=293K (open circles), T=313K(filled circles), T=333K (filled triangles), and T=353K (filled invertedtriangles).

FIG. 20B shows the temperature dependence of the transverse coefficientα_(E,31) for the multilayer with n=14. The lines are guides to the eye.Values of α_(E,31) are for the bias field H corresponding to maximumvalue in the ME effect. The temperature dependence is for a frequency of100 Hz.

FIG. 21 shows room temperature α_(E,31) at 100 Hz as a function of H formultilayers with n=10. The thickness of NFO and PZT layers wascontrolled to obtain a series of volume ratios of the magnetostrictive(v_(m)) to piezoelectric (v_(p)) phases, f=v_(m)/v_(p)=0.22–5.5. Thedata are for f=0.28 (circles), 0.37 (triangles), and 0.55 (diamonds).

FIG. 22 shows the variation of the transverse coefficient α_(E,31) withvolume ratio f=v_(m)/v_(p), for the multilayer. The solid linerepresents theoretical values for a two-layer structure. The values ofα_(E,31) are for bias field H corresponding to maximum value in the MEeffect and for a frequency of 100 Hz.

FIG. 23A shows a comparison of H dependence of theoretical transverse MEcoefficients estimated for a simple two-layer NFO-PZT structure with thedata for the bilayer and multilayer composites in FIGS. 18 and 19. Theopen (filled) circles are data points for the bilayer (multilayer). Thecrosses are the theoretical values estimated using Equations (4) and(5), v_(m)/v_(p)=1, material parameters given in Example 6, and theslope δλ/δH of magnetostriction λ vs H data in FIG. 17.

FIG. 23B shows a comparison of H dependence of theoretical longitudinalME coefficients estimated for a simple two-layer NFO-PZT structure withthe data for the bilayer and multilayer composites in FIGS. 18 and 19.The open (filled) circles are data points for the bilayer (multilayer).The crosses are the theoretical values estimated using Equations (4) and(5), v_(m)/v_(p)=1, material parameters given in Example 17, and theslope δλ/δH of magnetostriction λ vs H data in FIG. 17.

FIG. 24 shows a schematic view of an embodiment of magnetic field sensor10 comprising magnetostrictive layers 18 and 20 and piezoelectric layer12.

DETAILED DESCRIPTION OF THE INVENTION

All patents, patent applications, government publications, governmentregulations, and literature references cited in this specification arehereby incorporated herein by reference in their entirety. In case ofconflict, the present description, including definitions, will control.

The term “magnetoelectric” refers to the effect in which an electricfield is produced in a material when it is subjected to a staticmagnetic field or vice versa. It is either the dielectric polarizationof a material in an applied magnetic field or an induced magnetizationin an external electric field. Materials that respond to both electricand magnetic fields are considered to be magnetoelectric and tofacilitate field conversion.

The term “magnetostrictive” refers to the dimensional change by which aferromagnetic material is transformed from one shape to another in thepresence of a magnetic field. This solid-state phenomenon is a result ofthe rotation of small magnetic domains, causing internal strains in thematerial. These strains result in a positive expansion of the materialin the direction of the magnetic field. As the magnetic field isincreased, more domains rotate and become aligned until magneticsaturation is achieved. If the magnetic field is reversed, the directionof the domains is also reversed but the strains still result in aposition expansion in the field direction. Since the magnetostrictiveforces are molecular in origin, the mechanical response is very fast—amatter of microseconds. On the macroscopic scale, a magnetostrictivematerial conserves volume (of an essentially incompressible material);the diameter decreases as the length grows. The effect generates elasticforces in accordance with a generalized Hooke's law.

The term “piezoelectric” refers to the effect in those materials whichcontain certain crystals that exhibit charges. The crystals only producean electric output when they experience a change in load. Thepiezoelectric effect is exhibited by certain crystals, e.g., quartz andRochelle salt, and certain ceramic materials such as those of the leadzirconate titanate family and the barium titanate family. When a voltageis applied across certain surfaces of a solid that exhibits thepiezoelectric effect, the solid undergoes a mechanical distortion.Piezoelectric materials are used in transducers, e.g., phonographcartridges, microphones, and strain gauges, which produce an electricaloutput from a mechanical input, and in earphones and ultrasonicradiators, which produce a mechanical output from an electrical input.Piezoelectric solids typically resonate within narrowly definedfrequency ranges; when suitably mounted they can be used in electriccircuits as components of highly selective filters or asfrequency-control devices for very stable oscillators.

The term “Oe” refers to oersted which is measure of magnetic fieldstrength. One oersted is equal to 79.578 ampere per meter (A/m).Conversely, 1 A/m is 0.012566 Oe. The ampere per meter is theInternational Unit of magnetic field strength. It is derived from basicstandard units but is expressed directly in base units and cannot befurther reduced. Magnetic field strength is directly proportional to thelinear density so if the linear current density doubles, so does themagnetic field strength, if the linear density decreases by half, themagnetic field decreases by half.

Composites are desirable for the synthesis of materials with unique orimproved properties (Van Suchtelen, Philips Res. Rep., 27: 28 (1972)).Samples containing piezomagnetic and piezoelectric phases, for example,are product property composites capable of conversion of energies storedin electric and magnetic fields (Van den Boomgaard, et al., J. Mater.Sci. 9: 1705 (1974); Van den Boomgaard, et al., Ferroelectrics 14: 727(1976); Van den Boomgaard and Born, J. Mater. Sci. 13: 1538 (1978);Harshe et al., Int. J. Appl. Electromag. Mater. 4: 145 (1993);Avellaneda and Harshe, J. Intell. Mater. Syst. Struct. 5: 501 (1994);Lupeiko et al., Inorganic Materials 31: 1245 (1995); Ryu et al., J.Elec. Ceramics 7: 17 (2001); Ryu et al., Jpn. J. Appl. Phys. 40: 4948(2001); Srinivasan et al., Phys. Rev. B 64: 214408 (2001); Srinivasan etal., Phys. Rev. B 65: 1344xx (2002)). The field conversion is possiblesince an applied magnetic field produces a strain in the piezomagneticphase, which in turn is coupled to the piezoelectric phase, resulting inan induced electric field. The magnetoelectric (ME) coupling is studiedby measuring the induced electric field δE produced by an applied acmagnetic field δH. The ME voltage coefficient α_(E) is given byα_(E)=δE/δH.

Studies on ME composites were initiated in the early 1970s and wereprimarily on bulk samples of ferrimagnetic spinel ferrites andpiezoelectric barium titanate (Van den Boomgaard, et al., J. Mater. Sci.9: 1705 (1974); Van den Boomgaard, et al., Ferroelectrics 14: 727(1976); Van den Boomgaard and Born, J. Mater. Sci. 13: 1538 (1978)).Although ferrites are not piezomagnetic, magnetostriction in an acmagnetic field gives rise to pseudo piezomagnetic effects. The bulkcomposites yielded α_(E) values that were two to three orders ofmagnitude smaller than theoretical predictions. Such low values areprimarily due to low resistivity for ferrites that (i) limits theelectric field used for poling the composite and consequently a poorpiezoelectric coupling and (ii) a leakage current that results in theloss of induced voltage. These difficulties could easily be eliminatedin a layered composite (Harshe et al., Int. J. Appl. Electromag. Mater.4: 145 (1993); Avellaneda and Harshe, J. Intell. Mater. Syst. Struct. 5:501 (1994)). Theoretical estimates of α_(E) for a bilayer offerrite-lead zirconate titanate (PZT) are comparable to bulk compositevalues. The ease of poling in a bilayer allows enhancement ofpiezoelectric, and therefore, ME effects. The largest α_(E) everachieved was reported recently in trilayers of terfenol and PZT (Ryu etal., Jpn. J. Appl. Phys. 40: 4948 (2001)).

Ferrite (Fe₂O₄) based layered composites studied in the prior artinclude cobalt ferrite (CFO)- or nickel ferrite (NFO)-PZT (Harshe etal., Int. J. Appl. Electromag. Mater. 4: 145 (1993); Avellaneda andHarshe, J. Intell. Mater. Syst. Struct. 5: 501 (1994); Lupeiko et al.,Inorganic Materials 31: 1245 (1995); Srinivasan et al., Phys. Rev. B 64:214408 (2001) and Example 6). In particular, composites with CFO are ofinterest because of high magnetostriction. Samples were prepared eitherby laminating and sintering thick films of ferrites and PZT or by gluingferrite and PZT discs with silver epoxy (Harshe et al., Int. J. Appl.Electromag. Mater. 4: 145 (1993); Avellaneda and Harshe, J. Intell.Mater. Syst. Struct. 5: 501 (1994)). Studies on CFO-PZT structuresrevealed a maximum α_(E) on the order of 75 mV/cm Oe, an order ofmagnitude larger than in bulk composites. But the measured values,however, are an order of magnitude smaller than theoretical values for abilayer. Thus composites with magnetically hard cobalt ferrite showedweak ME coupling in spite of high magnetostriction (Harshe et al., Int.J. Appl. Electromag. Mater. 4: 145 (1993); Avellaneda and Harshe, J.Intell. Mater. Syst. Struct. 5: 501 (1994)). Nickel ferrite, on theother hand, is a soft ferrite with a much smaller anisotropy andmagnetostriction than CFO. In Example 6 using layered samples ofNFO-PZT, the results showed a large ME coupling on the order of 1500mV/cm Oe and α_(E) values in excellent agreement with theory. Theseobservations are indicative of the influence of magnetic parameters suchas anisotropy and initial permeability of ferrites on ME effects. Inparticular, the Joule magnetostriction that gives rise to pseudopiezomagnetic is dependent on the domain dynamics, which is a functionof anisotropy and coercivity.

Example 1 provides an understanding of the effects of the magneticparameters of ferrites on ME coupling in multilayers with PZT. It ispossible to accomplish controlled variations in such parameter with Zincsubstitution in ferrites. The results in Example 1 relate to thestrength of ME coupling in the bimetal ferrites wherein one of themetals of the bimetal is zinc-PZT composites of the present invention.The following oxides were used for the magnetic phase of the compositesof the present invention: cobalt zinc ferrite, Co_(1−x)Zn_(x)Fe₂O₄(CZFO) (x=0–0.6), nickel zinc ferrite Ni_(1−x)Zn_(x)Fe₂O₄ (NZFO)(x=0–0.5), and lithium zinc ferrite Li_(0.5−x/2)Zn_(x)Fe_(2.5−x/2)O₄(LZFO) (x=0–0.4). Commercially available PZT was used for thepiezoelectric phase. Layered samples were prepared by lamination andsintering of ferrite and PZT thick films obtained by tape casting. TheME voltage coefficients were measured for transverse (δE perpendicularto δH) and longitudinal (δE parallel to δH) field orientations forfrequencies 10–1000 Hz.

Example 1 provides clear evidence for strengthening of ME effects withthe substitution of Zinc for a part of the metal component in the metalferrite to produce the bimetal ferrite wherein one of the metals iszinc-PZT composites of the present invention, with the largest increasein CZFO-PZT and the smallest increase for NZFO-PZT. In all of thecomposites, a transverse ME coupling that is at least an order ofmagnitude stronger than the longitudinal coupling due to relativestrengths of the piezomagnetic effects was observed. In CZFO-PZT, α_(E)increased as the Zinc concentration was increased and showed a maximumfor x=0.4. The coupling weakened with further increase in zincconcentration. There was an excellent correlation between variations ofinitial permeability and ME coupling with x. Comparison of data andtheoretical estimates of α_(E) indicated very good agreement only forZinc substituted composites, x=0.4. A similar behavior for ME couplingwas observed for LZFO-PZT multilayer samples, but the maximum in α_(E)occurred for samples with x=0.3. But for NZFO-PZT, the theory accountedvery well for the magnitude and H variation of α_(E) for the entireseries of samples, irrespective of x value.

The data indicated that enhancement in ME coupling with zincsubstitution was related to (i) a reduction in magnetic anisotropy,leading to high permeability and, therefore, strong pseudo piezomagneticeffects in the ferrite and (ii) a reduction in processing temperatureresulting in fewer interface defects and efficient coupling between theferrite and PZT layers. The multilayer ferrite-PZT structures hereinshowed a strong ME coupling and are of interest for use as sensors andin high frequency devices.

Therefore, the present invention provides multilayer composites whichfacilitate the conversion of an electric field into a magnetic field ora magnetic field into an electric field. The composites comprise one ormore alternating phases of two types: a first phase which is amagnetostrictive phase which comprises a ferromagnetic material thatdeforms in an applied magnetic field and a second phase which is apiezoelectric phase which comprises a piezoelectric material thatconverts the deformation to an electric field.

The first phase comprises a sintered bimetal ferrite wherein one of themetals comprising the bimetal component is zinc (substituted with zinc),which deforms in a magnetic field. The second phase comprises apiezoelectric material such as lead zirconate titanate (PZT), leadzincate niobate (PZN), lead zincate niobate lead-titanate (PZN-PT), leadmagnesium niobate lead-titanate (PMN-PT), lead lanthanum zirconatetitanate (PLZT), Nb/Ta doped-PLZT, and barium zirconate titanate (BZT)which generates electricity when deformed. Preferably, the bimetalferrite wherein one of the metals comprising the bimetal component iszinc is selected from the group consisting of nickel zinc ferrite(NZFO), cobalt zinc ferrite (CZFO), lithium zinc ferrite (LZFO), andcombinations thereof.

In a preferred embodiment, the sintered bimetal ferrite wherein one ofthe metals comprising the bimetal component is zinc (Zn) has the formulaMetal_(1−x)Zn_(x)Fe₂O₄ wherein x is between about 0.1 to 0.6.Preferably, the metal is cobalt (Co), nickel (Ni), or lithium (Li). Whenthe bimetal ferrite comprises Co (CZFO), it has the formulaCo_(1−x)Zn_(x)Fe₂O₄ wherein X is between about 0.1 to 0.6, mostpreferably, between about 0.2 to 0.5. When the bimetal ferrite comprisesNi (NZFO), it has the formula Ni_(1−x)Zn_(x)Fe₂O₄ wherein x ispreferably between about 0.1–0.5. When the bimetal ferrite comprises Li(LZFO), it has the formula Li_(0.5−x/2)Zn_(x)Fe_(2.5−x/2)O₄ wherein x ispreferably between about 1.0 and 0.4.

The multilayer composites are made by separately producing films of eachof the two phases and arranging the films into a bilayer or into amultilayer comprising alternating layers of the two phases which arethen laminated and sintered at high temperature to produce the bilayeror multilayer composites of the present invention.

The bilayer and multilayer composites of the present invention show alarge magnetoelectric (ME) effect and maximum field conversionefficiency. The novel element of the composites is that zinc comprisesone of the metals in the bimetal ferrite. For example, the NZFO-PZTcomposites have a conversion coefficient (α_(E)) which ranges from about300 to 1500 mV/cm Oe depending on the concentration of zinc in the NZFOand the number of layers comprising the composite. The ME coefficient ismaximum at room temperature and a general increase in α_(E) is observedwith increasing frequency. The observed α_(E) values for the bilayer andmultilayer composites of the present invention are the largest evermeasured for a ferrite-PZT system. The reason for the large ME effectand maximum field conversion efficiency is that the bimetal ferritecomprises zinc as one of the metals in the bimetal component whichimparts a high piezomagnetic effect and an efficient stress-mediatedmagnetic moment-electric dipole coupling at the NZFO-PZT interface.CZFO-PZT, NZFO-PZT, and LZFO-PZT composites all have impressiveconversion coefficients (α_(E)) over metal ferrite composites. The α_(E)of the CZFO-PZT, NZFO-PZT, and LZFO-PZT composites depends on theconcentration of zinc in the bimetal ferrite and the number of layerscomprising the composite. It was particularly surprising that CZFO-PZTcomposites have a magnetoelectric effect that is about 600 times greaterthan that of CFO-PZT composites.

Bimetal ferrites wherein one of the metals is zinc-piezoelectricmaterial bilayers and multilayers include NZFO-PZT, CZFO-PZT, LZFO-PZT,or combinations thereof are preferably synthesized from films preparedby tape casting. The process involves (i) preparation of asubmicron-sized powder of the bimetal ferrite wherein one of the metalsis zinc and the piezoelectric material, (ii) preparing thick-film tapesof each from the powders by doctor-blade techniques, and (iii)lamination and sintering of bilayers and multilayers.

Bimetal ferrite powder wherein one of the metals is zinc is obtained bystandard ceramic techniques, which involves mixing the oxides orcarbonates of the metals followed by pre-sintering and final sintering.One method for preparing CZFO is as follows. A mixture is preparedcomprising cobalt oxide (CoO), zinc oxide (ZnO), and ferric oxide(Fe₂O₃) in appropriate amounts to produce Co_(1−x)Zn_(x)Fe₂O₄ wherein Xis between about 0.1 to 0.6, most preferably, between about 0.2 to 0.5.The mixture is then ball milled, dried, followed by calcinating at about800° C. to 850° C. for about one to three hours. NZFO and LZFO can beprepared in a similar manner to produce Ni_(1−x)Zn_(x)Fe₂O₄ wherein x ispreferably between about 0.1–0.5 and Li_(0.5−x/2)Zn_(x)Fe_(2.5−x/2)O₄wherein x is preferably between about 0.1 and 0.4. The piezoelectricmaterial includes any one of PZT, PZN, PZN-PT, PMN-PT, PLZT, Nb/Tadoped-PLZT, and BZT.

For tape casting (Mistler et al., In Tape Casting: Theory and Practice(American Ceramics Society, Westerville, Ohio, 2000), powders of thebimetal ferrite wherein one of the metals is zinc and the piezoelectricmaterial are each separately mixed with a solvent such as ethyl alcoholand a dispersant such as Blown Menhaden fish oil (available from RichardE. Mistler, Inc., Morrisville, Pa.) and ball milled for 24 hours,followed by a second ball mill with a plasticizer such as butyl benzylphthalate and a solution containing a binder such as polyvinyl butyralin an ether-alcohol mixture for 24 hours to produce a slurry of thebimetal ferrite wherein one of the metals is zinc and a slurry of thepiezoelectric material. The plasticizer enhances the elasticity of thefilms prepared by tape casting of the slurries and the wettability ofthe ball-milled powders by the solution.

The slurries are each separately cast into tapes of either the bimetalferrite wherein one of the metals is zinc or the piezoelectric materialon a carrier film such as silicon-coated mylar sheets using doctor-bladetechniques. That is, the slurries are cast into tapes on a carrier filmusing a tape caster consisting of a pair of stationarymicrometer-controlled blades and a moveable casting bed such as thatavailable from Richard E. Mistler, Inc. In general, it is possible toobtain 10×20 cm² tapes with a thickness within the range of about 10 to200 μm, preferably 15 to 30 μm thick films. The bimetal ferrite whereinone of the metals is zinc and PZT tapes are then arranged to obtain thedesired structure, a bilayer consisting of a bimetal ferrite wherein oneof the metals is zinc and a PZT tape or multilayers consisting ofalternating layers of NZFO and PZT tapes.

The bilayer or multilayer structures are laminated under high pressurebetween about 210 Kgf/cm² and 352 Kgf/cm² (3000 and 5000 psi) and underhigh temperature of about 400° K (about 100° C.). The plasticizerenables dense bilayer or multilayer structures to be made. The greenlaminated bilayer or multilayer structures are then heated at about1000° K (about 1050 to 1200° C.) for several hours to evaporate thebinder. The final sintering of the laminated bilayer or multilayer isthen carried out at between about 1400 and 1500° K.

Bilayers can be made with 200-μm-thick films of the bimetal ferritewherein one of the metals is zinc and the piezoelectric material.Multilayers consist of alternate layers of the bimetal ferrite whereinone of the metals is zinc and the piezoelectric material totaling n+1layers of the bimetal ferrite wherein one of the metals is zinc and nlayers of PZT (n=5 to 29, preferably 10 to 15), with a layer thicknessof 10 to 20 μm.

The bilayers or multilayers can be cut perpendicular to the laminates toproduce appropriately sized disks or billets and then ground. Contactsto the can be made by firing silver paste or depositing nickel fromsolution and then poling to the bilayers or multilayers in a silicon oilat 90 to 100° C. for 20 to 30 minutes in an electric field of 2.6 to 3kV/mm.

Solvents suitable for preparing the tapes of the bimetal ferrite whereinone of the metals is zinc and the piezoelectric material include loweralkyl alcohols such as ethyl alcohol.

Binders suitable for preparing the tapes of the bimetal ferrite whereinone of the metals is zinc and the piezoelectric material include but arenot limited to polyvinyl butyral, Acyloid B-72, and polypropylenecarbonate.

Dispersants/deflocculants suitable for preparing the tapes of thebimetal ferrite wherein one of the metals is zinc and the piezoelectricmaterial include but are not limited to Blown Menhaden fish oil andHypermer KD-1.

Plasticizers suitable for preparing the tapes of the bimetal ferritewherein one of the metals is zinc and the piezoelectric material includebut are not limited to n-butyl phthalate, dioctyl phthalate, butylbenzyl phthalate, propylene carbonate, and polyalkylene glycol.

Carrier films suitable for preparing the tapes of the bimetal ferritewherein one of the metals is zinc and the piezoelectric material includebut are limited to silicon coated MYLAR, MYLAR-A propylene, andnon-silicon release carriers.

Structural studies on the composites can be done with x-ray diffractionon multilayers, crushed powder samples, and samples with exposedinterface by etching away the top layer. Magnetostriction can bemeasured using the standard gage technique.

The magnetoelectric bilayer and multilayer composites are useful in aplurality of applications such as smart sensors for detecting magneticfields by measuring the electrical voltage produced; sensors formeasurements of rotation speed, linear speed, or acceleration, which isparticularly in the automotive industry; read-heads in storage deviceswhich convert bits in magnetic devices to electric signals and viceversa; storage media such as magnetoelectric media to store information;and, high frequency devices such as electric field control of magneticdevices and magnetic field control of electric devices. Themagnetoelectric composites are further useful in passive solid statemagnetic sensors (PSSM) which include speed sensors, flow sensors, andelectrical current sensors; electric generators; and magnetic-fieldsensors.

PSSM sensors are disclosed in U.S. Pat. No. 6,279,406 to Li et al. andpublished U.S. Application 2001/0040450 to Li et al. A PSSM sensoranticipated by the present invention typically includes amagnetoelectric bilayer or multilayer composite comprising apiezoelectric material such as PZT, PZN, PZN-PT, PMN-PT, PLZT, Nb/Tadoped-PLZT, or BZT and a magnetostrictive material which is CZFO, NZFO,or LZFO. In response to a magnetic field, the magnetostrictive materialcauses a strain on the piezoelectric material that in turn produces anelectrical output signal. The PSSM sensors consume no electric power andproduce an electric signal with a magnetic field sensitivity of greaterthan 10 mV/Oe. PSSM sensors comprising the magnetoelectric multilayerand bilayer composites of the present invention can be mass produced atlow cost. Compared to variable reluctance coil sensor-based speedsensors, PSSM sensors comprising the magnetoelectric multilayer andbilayer composites of the present invention would be able to detect nearzero speeds and compared to Hall and magnetoresistance sensors, hasbetter field sensitivity, better temperature stability, and are lessexpensive to produce. When used as flow sensors, PSSM sensors comprisingthe magnetoelectric multilayer and bilayer composites of the presentinvention are particularly useful because they would in general requireno power source and offer better sensitivity and smaller size than othermagnetic sensors. When used as electrical current sensors, PSSM sensorscomprising the magnetoelectric multilayer and bilayer composites of thepresent invention are particularly useful because they would have littleor no zero offset voltage and would require little or no temperaturecompensation for general use.

FIG. 24 illustrates an embodiment of a magnetic field sensor forestimating a value of a static or dynamic magnetic field as disclosed inpublished U.S. Application 2001/0028245 to Li et al. Such magnetic fieldsensors can include the magnetoelectric composites of the presentinvention. By way of example, FIG. 24 shows a magnetic field sensor 10comprising a piezoelectric layer 12 comprising a piezoelectric materialsuch as PZT, PZN, PZN-PT, PMN-PT, PLZT, Nb/Ta doped-PLZT, or BZTpositioned between and mechanically connected across interfaces 14 and16 to magnetostrictive layers 18 and 20 comprising either CZFO, NZFO, orLZFO. The interfaces 14 and 18 can be a thin electrically insulatinglayer or no insulating layer at all. A charge carrying line 22 isconnected to the piezoelectric layer 12 at first and second spaced apartlocations, 24 and 26, and a constant current source 28 is positioned online 22. A voltmeter or other electrical measurement meter 30 is alsopositioned to measure a voltage difference or similar electricalmeasurement value created between third and fourth spaced apartpositions, 32 and 34, on piezoelectric material 12 in response to thepassage of current. Further configurations are shown in published U.S.Application 2001/0028245 to Li et al. For example, the sensor is abilayer comprising a piezoelectric layer and a magnetostrictive layermechanically connected at interface or the sensor comprises amagnetostrictive layer positioned between two piezoelectric layers andmechanically connected across the interfaces therebetween.

When the magnetostrictive layers, 18 and 20, are exposed to a magneticfield H, the layers will attempt to expand or contract in a selecteddirection that depends upon a present value H and a reference valueH(ref) of the magnetic field. This difference in length in a selecteddimension of the magnetostrictive layers, 18 and 20, produces a strainacross interfaces 14 and 16. The attempt to change the length of themagnetostrictive layers, 18 and 20, may be partly or wholly resisted bythe piezoelectric layer 12, interfaces, 14 and 16, or both, with theresult that the piezoelectric layer 12 and the interfaces, 14 and 16,experience a strain, stress, or both. The induced strain in thepiezoelectric layer 12 causes a change in the resistivity of thepiezoelectric layer 12 which will in turn cause a change in theresistance associated with the path taken by the current through thepiezoelectric layer 12 between the third and fourth locations, 32 and34. This change in resistance will register as a change in currentpassing along line 22, or as a change in a voltage difference betweenthe first and second locations, 24 and 26, and the change in current orvoltage difference will register as a change in a reading of an ammeteror voltmeter 30. A change in current or voltage is measured for constantvoltage or current, respectively.

The bilayer and multilayer composites of the present invention are alsouseful in the following specific applications: filtering elements usedin conjunction with electrical connectors to suppress electromagneticinterference as disclosed in U.S. Pat. No. 5,512,196 to Mantese et al.,piezomagnetometers such as those disclosed in U.S. Pat. No. 5,675,252 toPodney for detecting and measuring magnetic and electric fields,magnetometers disclosed in U.S. Pat. Nos. 4,769,599 and 5,130,654 toMermelstein, and memory storage material such as that used in computersand disclosed in U.S. Pat. No. 5,390,142 to Gendlin.

The following examples are intended to promote a further understandingof the present invention.

EXAMPLE 1

This example provides an understanding of the effects of the magneticparameters of ferrites on ME coupling in multilayers with PZT. It ispossible to accomplish controlled variations in such parameter with Zincsubstitution in ferrites. The following oxides were used for themagnetic phase: cobalt zinc ferrite, Co_(1−x)Zn_(x)Fe₂O₄ (CZFO)(x=0–0.6), nickel zinc ferrite Ni_(1−x)Zn_(x)Fe₂O₄ (NZFO) (x=0–0.5), andlithium zinc ferrite Li_(0.5−x/2)Zn_(x)Fe_(2.5−x/2)O₄ (LZFO) (x=0–0.4).Commercially available PZT was used for the piezoelectric phase.

I. MATERIALS AND METHODS

Layered composites were synthesized using thick films of ferrites andPZT obtained by tape casting (Mistler and Twiname, Tape Casting: Theoryand Practice, The American Ceramics Society, Westerville, Ohio (2000)).The ferrite powder necessary for tape casting was prepared by thestandard ceramic techniques that involved mixing the oxides orcarbonates of the constituent metals, followed by pre-sintering, andfinal sintering. A ballmill was used to grind the powder to submicronsize. Commercially available PZT powder was used for the PZT films (PZT:sample No. APC850, American Piezo Ceramics, Inc., Mackeyville, Pa.). Acast of ferrite or PZT was made by ballmilling the powder in ethylalcohol with a dispersant, plasticizer, and a binder. The slurry thusobtained was cast into 15–30 micron thick tapes on mylar substratesusing a custom-made tape caster consisting of a pair of stationaryblades and a moveable platform. The tapes were dried for a day, removedfrom the substrate, and stacked to form the desired multilayer structureconsisting of alternate layers of ferrites and PZT. Samples with (n+1)layers of ferrites and n layers of PZT (n=10–15) were laminated at hightemperatures and high pressure and then sintered at 1375–1450 K forseveral hours. The two outer layers were ferrites in order to avoid anyunwanted reaction between PZT and aluminum oxide powder used in thesintering process.

X-ray diffraction studies on the samples did not show any evidence forany phases other than spinel ferrite and PZT. Saturation magnetizationmeasured with a Faraday balance was in agreement with expected valuesfor ferrites. Magnetostriction was measured with the standard straingage technique. The samples (5 mm×5 mm×0.5 mm) were then polished andpoled in an electric field. The poling procedure involved heating thesample to 500 K and cooling it back to room temperature in an electricfield of 15–30 kV/cm applied perpendicular to the sample plane.Electrodes were deposited on the sample with silver paint. Thepiezoelectric coupling coefficient was measured with a d₃₃-meter. Formagnetoelectric characterization, the samples were placed in a shielded3-terminal sample holder and placed between the pole pieces of anelectromagnet that was used to apply a static magnetic field H. Therequired ac magnetic field (10 Hz–1 kHz) δH parallel to H was generatedwith a pair of Helmholtz coils. The resulting ac electric field δEperpendicular to the sample plane (direction-3) was estimated frommeasured voltage (with a lock-in-amplifier). The transverse coefficientα_(E,31) was measured for the magnetic fields (along direction-1)parallel to the sample plane and perpendicular to δE. The longitudinalcoefficient α_(E,33) was measured for all the fields perpendicular tothe sample plane. Magnetoelectric characterization was carried out as afunction of frequency of the ac magnetic field, bias magnetic field Hand sample temperature.

II. RESULTS

Studies were performed on multilayers with equal thickness for ferriteand PZT layers (15–30 micron) and a series of n-values (the number ofPZT layers). The results reported here are for samples with n=10–15.

First to be considered were samples of CZFO-PZT. FIG. 1 showsrepresentative data on the H dependence of α_(E,31) and α_(E,33) for asample in which 40% Co was replaced by Zinc. The data at roomtemperature and 1 Oe ac field at 100 Hz was for a sample with n=10. Asthe bias field was increased from zero, a rapid increase to a peak valuewas observed for α_(E). With a further increase in H, the ME couplingcoefficients dropped to a minimum or zero value. When H was reversed, a180 degree phase difference relative to the ME voltage for +H and asmall decrease in the peak value for α_(E) compared to the value for +Hwere observed. There was no noticeable hysteresis or remenance in theα_(E) vs H behavior. As discussed later, the H dependence in FIG. 1essentially tracked the strength of piezomagnetic coupling which wasproportional to the rate of change in magnetostriction with H in theferrite. The coupling vanished when magnetostriction attainedsaturation.

Now the H dependence of transverse and longitudinal coefficients wascompared. Although overall features for variations with H were similarfor both cases, the following differences were found: (i) the initialrate of increase in α_(E) with H was much higher for the transverse casethan for longitudinal orientation for the fields, (ii) the peak α_(E,31)was a factor of five higher than α_(E,33), and (ii) the peak value inα_(E,33) occurred for a higher bias field H than for the transversecase. These observations can be understood in terms of H variation ofparallel and perpendicular magnetostriction for the ferrite.

Similar α_(E) vs H data were obtained for samples with x-values varyingfrom 0 to 0.6. Both α_(E,31) and α_(E,33) were measured. FIG. 2 showsthe room temperature variation of α_(E,31) with the bias magnetic fieldfor x=0–0.4. Data on the longitudinal coupling are not shown since thecoupling was quite weak for all x-values except for 0.4. As x wasincreased, the rate at which α_(E,31) varies with H at low staticmagnetic fields increased, (ii) the peak in α_(E,31) occurred atprogressively decreasing H, and (iii) there was a general increase inthe peak value of α_(E,31).

In FIG. 3, the variation of peak values of α_(E) with x is shown forboth transverse and longitudinal coefficients. As the Zinc substitutionwas increased, a sharp increase in α_(E,31), from 50 mV/cm Oe for x=0 to280 mV/cm Oe for x=0.4 was observed. A further increase in x wasaccompanied by a substantial reduction in α_(E,31). A similar characterwas evident for the longitudinal coupling parameter.

Past attempts using cobalt ferrite-based ME composites included bulksamples with barium titanate or PZT and multilayers with PZT (Harshe,Magnetoelectric effect in piezoelectric-magnetostrictive composites, PhDthesis, The Pennsylvania State University, College Park, Pa., (1991)).Bulk samples of CFO-barium titanate showed evidence for ME coupling, butreported α_(E) values were a factor of 3–5 smaller than values in FIG.3. Bulk composites of CFO-PZT showed very weak ME effects, but layeredsamples showed a maximum α_(E) of 75 mV/cm Oe (Harshe, Magnetoelectriceffect in piezoelectric-magnetostrictive composites, PhD thesis, ThePennsylvania State University, College Park, Pa., (1991)). It is clearfrom the data in FIGS. 1–3 that Zinc substitution in cobalt ferrites wasa key ingredient for strong ME coupling in multilayers. The efficientfield conversion properties can be attributed to modification ofmagnetostriction due to Zinc substitution. There is also strong evidencethat Zinc facilitates strong coupling at ferrite-PZT interface.

Similar ME studies were performed on nickel zinc ferrite-PZT sampleswith x=0–0.5 and lithium zinc ferrite samples with x=0–0.4. FIGS. 4A and4B show representative data on the H dependence of α_(E) for NZFO-PZTsamples with x=0–0.4. The data were obtained on samples with n=10–15 atroom temperature for a frequency of 100 Hz. Data in FIGS. 4A and B arequalitatively similar to the observations for the composites ofCZFO-PZT. For NFO-PZT (x=0), α_(E,31) vs H data showed the expectedresonance-like character with a maximum centered at H=400 Oe. When Zincwas substituted for Ni, there was an increase in the peak value ofα_(E,31) for low x-values. A down-shift was observed in the biasmagnetic field corresponding to the peak value in of α_(E,31) as x wasincreased. The H field-range for strong ME effects decreases withincreasing Zinc content. Data on the longitudinal coupling parameter inFIGS. 4A and 4B show the following important departures from thetransverse case: (i) the coupling strength did not show any dependenceon x for low Zinc substitution, (ii) as x was increased, an up-shift wasobserved for the H-value corresponding to peak α_(E,33), and (iii) theME coupling was realized over a wide field range. Similar α_(E) vs Hprofiles were obtained for other x values.

The variations of peak values of α_(E) with x are plotted in FIG. 5. Thedata reveal a 60% increase in the transverse ME voltage coefficient as xwas increased from 0 to 0.2, followed by a reduction in α_(E,31) forhigher x. The longitudinal coefficient also showed a similar decreasewith increasing x for x>0.2. The present values of α_(E) for theNZFO-PZT layered samples were significantly higher than reported valuesin past studies on bulk or layered samples. The coupling coefficientmust be compared with 80 mV/cm Oe for NFO-barium titanate (Van denBoomgaard, et al., J. Mater. Sci. 9: 1705 (1974); Van den Boomgaard, etal., Ferroelectrics 14: 727 (1976); Van den Boomgaard and Born, J.Mater. Sci. 13: 1538 (1978)), 115 mV/cm Oe for NFO-PZT bulk composites(Ryu et al., J. Elec. Ceramics 7: 17 (2001)), and 300–400 mV/cm Oe forNFO-PZT bilayers and multilayers (Lupeiko et al., Inorganic Materials31: 1245 (1995); Srinivasan et al., Phys. Rev. B 64: 214408 (2001) andExample 6).

Finally, lithium zinc ferrite-PZT composites were analyzed. Samples withn=10 and 15, a layer thickness of 15 micron and ferrites of compositionLi_(0.5−x/2)Zn_(x)Fe_(2.5−x/2)O₄ for x=0–0.4 were synthesized. FIG. 6Ashows data on H-variation of the transverse ME voltage coefficient forx=0–0.3. The data are for a frequency of 100 Hz at room temperature.These data constitute the first report of strong ME coupling in lithiumferrite-PZT composites of any kind. Important observations were asfollows: (i) data showed features similar to the other two systems, (ii)a factor of five increase in the peak α_(E,31) value was evident when xwas increased from 0 to 0.3, and (iii) an up-shift in the H-valuecorresponding the peak α_(E,31) was seen as x was increased from 0(recall that for CZFO-PZT and NZFO-PZT samples, a down-shift in H formaximum α_(E,31) was observed for increasing x), and (iv) the H-intervalfor strong ME effects was essentially independent of x. FIG. 6B showsdata on peak values of α_(E,31) vs x. The Figure shows that there was arapid increase in the ME voltage coefficient with x for x=0–0.3,followed by a sharp decrease for x=0.4.

Now the ME voltage coefficient data for the multilayer structures werecompared and contrasted. Although all the three systems showedenhancement in the strength of ME coupling with Zinc substitution in theferrite phase, the largest increase occurs for CZFO-PZT samples,followed by LZFO-PZT and NZFO-PZT. The highest ME voltage coefficientswere measured for Zinc substitutions in the range x=0.2–0.4, dependingon the nature of ferrite. The ME coupling was present over a wideH-interval in CZFO-PZT compared to the other two samples. A dramaticshift in the H-value corresponding to peak α_(E,31) for CZFO-PZT wasobserved. The field shift in the other two samples was relatively small.The strongest ME coupling was observed in NZFO-PZT where as LZFO-PZTsamples showed the weakest effect.

III. DISCUSSION

It is reasonable to interpret the data in FIGS. 1–6 in terms ofvariations in magnetic properties of the ferrite since the resultsdescribed in he results section are on H dependence of the ME coupling.Recall that the sample and materials parameters for the piezoelectricoxide remained the same for all the composites. It is clear that oneneeds to focus in on changes in the magnetic parameters of the ferriteswhen Zinc replaces either 3-d ions or Li. We first estimate themagnetoelectric voltage coefficients and their bias magnetic fielddependence for comparison with data. Following this, we discuss thepossible cause of zinc substitution related enhancement in ME effects.

For theoretical calculation of α_(E), a basic bilayer structure offerrite-PZT is considered. Assuming the bilayer to be a free body withperfectly bonded interface with uniform H and zero electric field in theferrite, one obtains the following expressions for the transverse andlongitudinal ME coefficients (Harshe et al., Int. J. Appl. Electromag.Mater. 4: 145 (1993); Avellaneda and Harshe, J. Intell. Mater. Syst.Struct. 5: 501 (1994); Harshe, Magnetoelectric effect inpiezoelectric-magnetostrictive composites, PhD thesis, The PennsylvaniaState University, College Park, Pa., (1991)):α_(E,31) =δE ₃ /δH ₁=−2d ^(p) ₃₁ q ^(m) ₁₁(s ^(m) ₁₁ +s ^(m) ₁₂)ε^(T,p)₃₃+(s ^(p) ₁₁ +s ^(p) ₁₂)ε^(T,p) ₃₃−2(d ^(p) ₃₁)²  (1)andα_(E,33) =δE ₃ /δH ₃=−2d ^(p) ₃₁ q ^(m) ₁₃(s ^(m) ₁₁ +s ^(m) ₁₂)ε^(T,p)₃₃+(s ^(p) ₁₁ +s ^(p) ₁₂)ε^(T,p) ₃₃−2(d ^(p) ₃₁)²  (2)Here m denotes the magnetostrictive phase and p the piezoelectric phase,d and q are the piezoelectric and piezomagnetic coupling coefficients,respectively, s is the compliance coefficient and ε^(T) is permittivityat constant stress. Equations (1) and (2) are valid for equal volumes ofm-phases and p-phases and are for unit thickness of PZT. The voltagecoefficients α_(E,31) and α_(E,33) arise due to parallel (q₁₁) andperpendicular (q₁₃) piezomagnetic coupling constants, respectively. Thusone requires the magnitude of q=δλ/δH (where λ is the magnetostriction)and its variation with H for the estimation of filed dependence ofα_(E). The perpendicular magnetostriction, λ₁₃ and its derivative withH, q₁₃ were quite small. Consequently, the longitudinal ME coupling isexpected to be weaker than the transverse case, as is the case from datain FIG. 1–5. The discussion to follow is therefore restricted to thetransverse ME effect.

The q-values were determined from data on λ vs H for the pure and Zincsubstituted ferrites. Representative data on the in-plane parallelmagnetostriction λ₁₁ used for q₁₁ are shown in FIGS. 7A, 7B, and 7C forCZFO, NZFO and LZFO bulk samples, respectively. The measurements weremade with the standard strain gage technique at room temperature onferrites (1 cm×1 cm×0.05 cm) made from thick films.

Consider first the magnetostriction for CZFO samples (FIG. 7A). For x=0,as H was increased there was a weak increase in the magnitude of λ forfields up to 2 kOe which was followed by a strong increase for H overthe interval 2–4 kOe. The λ-value leveled off and became saturated forH>4 kOe. When 20% Co was replaced with Zinc, there was a dramaticstrengthening of low field piezomagnetic coupling in the ferrite. Themagnetostriction increased rapidly with H for fields up to 2 kOe. Thesaturation λ was the same as for x=0, but the saturation occurred at amuch smaller H compared to CFO. A further increase in x resulted in adecrease in the saturation λ, but q remained high since the saturationfield decreased progressively with increasing x. Thus with theintroduction of Zinc in CFO, there was an increase in the strength ofpiezomagnetic coupling and a reduction in the saturation value of λ.FIG. 7D shows that at x=0.4, λ₁₁ decreased as H was increased.

The substitution of Zinc in NFO or LZFO resulted in changes that wereless dramatic than those for CFO substituted with Zinc. Data in FIG. 7Bshow an overall decrease in the saturation λ and H value for saturationwith increasing x in NZFO. For x=0.2, there was an increase in the lowfield piezomagnetic coupling (δλ/δH) compared to NFO.

Data for LZFO (FIG. 7C) indicated no changes in the saturation λ withincreasing Zinc substitution for x=0–0.3. But both the low field λ and qincreased with increasing x. The highest λ was measured for CZFO and thelowest for LZFO.

The bilayer model can be used for a theoretical calculation of α_(E,31)for comparison with the data. The following material parameters wereused for the constituent phases (Harshe et al., Int. J. Appl.Electromag. Mater. 4: 145 (1993); Avellaneda and Harshe, J. Intell.Mater. Syst. Struct. 5: 501 (1994); Srinivasan et al., Phys. Rev. B 64:214408 (2001) and Example 6; Landolt-Börnstein: Numerical data andfunctional relationships in science and technology, Group III, Crystaland Solid State Physics, vol 4(b), Magnetic and Other Properties ofOxides, eds. Hellwege and Springer, Springer-Verlag, New York (1970)).

PZT, s^(p) ₁₁ = 14 × 10⁻¹² m²/N Ferrites, s^(m) ₁₁ = 6.5 × 10⁻¹² m²/Ns^(p) ₁₂ = −8 × 10⁻¹² m²/N s^(m) ₁₂ = −2.4 × 10⁻¹² m²/N. ε^(T, p) ₃₃ =17 × 10⁻⁹ F/m.Measured values of the piezoelectric coupling coefficient d₃₃ for themultilayers ranged from 70 to 170 pm/V at 100 Hz and an average value ofd₃₁=d₃₃/2=60 pm/V was used. The other required parameter, q, wasdetermined from data in FIGS. 7A, 7B, 7C, and 7D. Calculated values ofα_(E,31) are compared in FIGS. 8A, 8B, and 8C with the data for CZFO-PZTsamples. Results are shown for samples with x=0, 0.2 and 0.4. ForCFO-PZT (x=0), the theory predicted a gradual increase in α_(E,31) withincreasing H. A maximum in α_(E,31) was expected for a field of 3 kOeand the ME coefficient drops down to zero value for H=5 kOe. Uponincreasing x from 0 to 0.2, significant theoretical predictionsconcerned a rapid increase in the low field α_(E,31), down-shift in thepeak position to 400 Oe and a maximum α_(E,31) value that was 25% largerthan for x=0. Finally, for x=0.4, the calculated α_(E,31) vs H revealeda further shift to low H for the peak in α_(E,31), a rapid fall-off inα_(E,31) at higher fields, and a maximum α_(E,31) which was a factor of4–5 smaller than for the cobalt rich compositions. The theoreticalα_(E,31) vs H tracked the slope of λ vs H as shown in FIGS. 7A, 7B, 7C,and 7D.

The data and theoretical values of α_(E,31) for CZFO-PZT were compared.For x=0–0.2, there was a substantial disagreement between theory anddata. Neither the magnitude nor the H dependence of calculated α_(E,31)agreed with the data. The predicted values were an order of magnitudehigher and H-values for maximum α_(E,31) were a factor of 2–3 smallerthan the measured values. The most significant inference from FIGS. 8A,8B, and 8C was the agreement between theory and data only for x=0.4(FIG. 8C). In particular, theoretical and measured α_(E,31) values wereidentical over the field interval 0–300 Oe. For higher fields, themeasured values were somewhat higher than the theoretical values. Thedata shows a maximum in α_(E,31) for a H-value that was slightly higherthan predicted by theory.

A similar comparison for NZFO-PZT samples, however, indicated excellentagreement between theory and data for pure nickel ferrite and the entireseries of Zinc substitution. Representative results are shown in FIGS.9A and 9B for NFO(x=0)-PZT. Data on α_(E,31) are shown for bothincreasing and decreasing H. Theoretical estimates are for q-valuesobtained from magnetostriction data in FIGS. 7A, 7B, and 7C and othermaterial parameters mentioned earlier for ferrites. Both the magnitudeand H dependence agreed with the data. The theory predicted a maximum inα_(E,31) which was 20% higher than the data and the maximum occurred foran H-value lower than the observed field.

Finally, a comparison of data and theory in FIGS. 9A and 9B for LZFO-PZTsamples yielded results similar to CZFO-PZT samples. For x=0, there wasa factor of six difference between the calculated and experimentalvalues of α_(E,31). When Zinc replaced both Fe and Li, there was overallagreement in α_(E,31) values as evident from results in FIGS. 9A and 9Bfor x=0.3.

There are several common features in the theoretical estimates for thethree systems. First, for composites containing pure ferrites, such asCFO-PZT or LFO-PZT, there was a total lack of agreement between theoryand data. Although magnetostriction data implied a strong piezomagneticcoupling, the magnetoelectric coupling was almost an order of magnitudeweaker than predicted values. The notable exception, however, wasNFO-PZT which showed good agreement. Second, the introduction of Zincled to an enhancement in the strength of ME coupling and there was goodagreement between theory and data for Zinc rich compositions of bothCZFO-PZT and LZFO-PZT samples. Third, the theoretical value of Hcorresponding to the peak in α_(E,31) was smaller than the measuredvalues for all the compositions. Fourth, the high field values ofα_(E,31) were smaller than the data. The final two observations couldhave been the result of an over simplified model that was meant for abilayer structure.

Example 6 provides an analysis of ME coupling in bilayer and multilayerNFO-PZT. The effective thickness of NFO and PZT was kept constant forboth samples. There was a down-shift in H corresponding to the maximumin α_(E,31) and a sharp drop in high field α_(E,31) for the bilayercompared to multilayers (Srinivasan et al., Phys. Rev. B 64: 214408(2001) and Example 6). It was important to examine the influence offerrite-PZT interfaces in the theoretical model since there were ninterfaces in a multilayer with (n+1) ferrites and n PZT layers. Onewould expect a stronger interface coupling in multilayers than inbilayers.

Consider next that the lack of agreement between theory and data forpure or low-Zn compositions of CZFO-PZT and LZFO-PZT implied weak MEcoupling mediated by magnetostriction. There are two types ofmagnetostriction in a ferromagnet: (i) Joule magnetostriction associatedwith domain movements and (ii) volume magnetostriction associated withmagnetic phase change. The volume magnetostriction is not important inthe present situation since it is significant only at temperatures closeto the Curie temperature and for high applied fields. In ferrites,domains are spontaneously deformed in the magnetization direction. Underthe influence of a bias field H and ac field δH, both the growth in thedomains with favorable orientation and domain rotation contribute to theJoule magnetostriction. Since the ME coupling involves dynamicmagnetoelastic coupling, key requirements for the ferrite phase areunimpeded domain wall motion, domain rotation and a large λ. A soft,high initial permeability (low anisotropy) ferrite, such as NFO, is themain ingredient for strong ME effects. In magnetically hard cobaltferrite, however, one has the disadvantage of a large anisotropy fieldthat limits domain rotation. Our magnetization measurements yielded aninitial permeability of 20 for NFO vs 3.5 for CFO. Thus one can relatethe strong ME effects in NFO-PZT and poor ME coupling in CFO-PZT to theinitial permeability μ_(i). With the substitution of Zinc in CFO, theanisotropy decreased and μ_(i) increased. FIG. 10 shows the compositiondependence of μ_(i) in CZFO. With an increase in x, μ_(i) (fromToltinnikov and Davydov, Soviet Phys. Solid State 6: 1730 (1965))increased and had a maximum value when x=0.5. FIG. 10 also shows thevariation of maximum α_(E,31) with x for comparison. There was excellentcorrelation between the initial permeability and the strength of MEcoupling. The maximum in both μ_(i) and α_(E,31) occurred around thesame composition, i.e., x=0.4–0.5. Thus it is logical to associate theagreement between theory and data in FIGS. 8A, 8B, and 8C for Zn-richCZFO-PZT with efficient dynamic magnetoelastic coupling that resultedfrom domain mobility. The bilayer model used here, however, does notpredict a direct relationship between μ_(i) and α_(E,31) (Harshe et al.,Int. J. Appl. Electromag. Mater. 4: 145 (1993); Avellaneda and Harshe,J. Intell. Mater. Syst. Struct. 5: 501 (1994)). Recent theoreticalmodels for multilayer ferrite-PZT composites do reveal the anticipateddependence of ME coupling on initial permeability.

Zinc substitution can also influence the coupling at the ferrite-PZTinterface. Since the ME effect is an interface phenomenon, coupling islikely to be adversely affected by any defects, either structural orchemical, and growth induced strain. Defects and strain increasemagnetic anisotropy, pin the domain and limit wall motion and rotation.Zinc substituted composites were sintered at a lower temperaturecompared to pure ferrite-PZT samples, possibly resulting in fewerinterface defects and reduced strain. Investigations on the microscopicnature of the ferrite-PZT interfaces with techniques such as electronmicroscopy and magnetic force microcopy are critically important for anunderstanding of the Zinc substitution related interface aspects of theobservations.

IV. CONCLUSIONS

Studies on layered samples of zinc substituted ferrites and PZT showedefficient field conversion characteristics. The magnetoelectric voltagecoefficient α_(E) measured from induced voltage for an applied magneticfield showed an overall increase with increasing Zinc concentration (x)in CZFO-PZT, NZFO-PZT and LZFO-PZT samples. A maximum in α_(E) occurredfor x=0.2–0.4, depending on the ferrite. Theoretical estimates of α_(E)were in very good agreement with data only for NZFO-PZT and Zinc richcompositions of CZFO-PZT and LZFO-PZT. There was excellent correlationbetween α_(E) and initial permeability for CZFO-PZT. The Zinc assistedenhancement in ME coupling is attributed to low anisotropy and highpermeability for the ferrites that results in favorable domain dynamicsand strong piezomagnetic coupling in the composites.

EXAMPLE 2

This example illustrates the formation of nickel zinc ferrite-leadzirconate titanate (NZFO-PZT) bilayers and multilayers. Both NZFO-PZTbilayers and multilayers were synthesized from thick films prepared bytape casting. The process involved (i) preparation of submicron-sizedpowder of nickel zinc ferrite (NZFO) and lead zirconate titanate (PZT),(ii) thick-film tapes by doctor-blade techniques, and (iii) laminationand sintering of bilayers and multilayers.

NZFO powder was obtained by standard ceramic techniques and commercialPZT (Sample No. APC850) was obtained from American Piezo Ceramics, Inc.,Mackeyville, Pa.) were used. For tape casting (Mistler et al., In TapeCasting: Theory and Practice (American Ceramics Society, Westerville,Ohio, 2000), powders of NZFO and PZT were each mixed with a solvent suchas ethyl alcohol and a dispersant such as Blown Menhaden fish oil(available from Richard E. Mistler, Inc., Morrisville, Pa.) and ballmilled for 24 hours, followed by a second ball mill with a plasticizersuch as butyl benzyl phthalate and a binder such as polyvinyl butyralfor 24 hours to produce a slurry of NZFO and a slurry of PZT. Theslurries were each separately cast into tapes of either NZFO or PZT on acarrier film such as silicon-coated mylar sheets using doctor-bladetechniques. That is the slurries were cast into tapes on a carrier filmusing a tape caster consisting of a pair of stationarymicrometer-controlled blades and a moveable casting bed such as thatavailable from Richard E. Mistler, Inc. In general, it was possible toobtain 10×20 cm² tapes with a thickness within the range of about 10 to200 μm. The NZFO and PZT tapes were then arranged to obtain the desiredstructure, a bilayer consisting of an NZFO and a PZT tape or multilayersconsisting of alternating layers of NZFO and PZT tapes. The bilayer ormultilayer structures were laminated under high pressure between about210 Kgf/cm² and 352 Kgf/cm² (3000 and 5000 psi) and under hightemperature of about 400° K. The laminated bilayer or multilayerstructures were then heated at about 1000° K to evaporate the binder.The final sintering of the laminated bilayer or multilayer was thencarried out at between about 1400 and 1500° K.

Bilayers were made with 200-μm-thick films of NZFO and PZT. Multilayersconsisted of alternate layers of NZFO and PZT totaling n+1 layers ofNZFO and n layers of PZT (n=5 to 29), with a layer thickness of 10 to 20μm. Structural studies were done with x-ray diffraction on multilayers,crushed powder samples, and samples with exposed interface by etchingaway the top layer. There was no evidence of any impurity phase.Magnetic parameters such as the saturation magnetism, anisotropy, and gvalue were in agreement with values expected. Thus the high temperatureprocessing did not result in any impurity phases or degradation of thequality of the magnetic phase.

EXAMPLE 3

This example illustrates the formation of cobalt zinc ferrite-leadzirconate titanate (CZFO-PZT) bilayers and multilayers. Both CZFO-PZTbilayers and multilayers were synthesized from thick films prepared bytape casting. The process involved (i) preparation of submicron-sizedpowder of cobalt zinc ferrite (CZFO) and lead zirconate titanate (PZT),(ii) thick-film tapes by doctor-blade techniques, and (iii) laminationand sintering of bilayers and multilayers.

CZFO powder was obtained by standard ceramic techniques and commercialPZT (Sample No. APC850) was obtained from American Piezo Ceramics, Inc.,Mackeyville, Pa.) were used. For tape casting, powders of CZFO and PZTwere each mixed with a solvent such as ethyl alcohol and a dispersantsuch as Blown Menhaden fish oil and ball milled for 24 hours, followedby a second ball mill with a plasticizer such as butyl benzyl phthalateand a binder such as polyvinyl butyral for 24 hours to produce a slurryof CZFO and a slurry of PZT. The slurries were each separately cast intotapes of either CZFO or PZT on a carrier film such as silicon-coatedmylar sheets using doctor-blade techniques. That is the slurries werecast into tapes on a carrier film using a tape caster consisting of apair of stationary micrometer-controlled blades and a moveable castingbed. In general, it was possible to obtain 10×20 cm² tapes with athickness within the range of about 10 to 200 μm. The CZFO and PZT tapeswere then arranged to obtain the desired structure, a bilayer consistingof an CZFO and a PZT tape or multilayers consisting of alternatinglayers of CZFO and PZT tapes. The bilayer or multilayer structures werelaminated under high pressure between about 210 Kgf/cm² and 352 Kgf/cm²(3000 and 5000 psi) and under high temperature of about 400° K. Thelaminated bilayer or multilayer structures were then heated at about1000° K to evaporate the binder. The final sintering of the laminatedbilayer or multilayer was then carried out at between about 1400 and1500° K.

Bilayers were made with 200-μm-thick films of CZFO and PZT. Multilayersconsisted of alternate layers of CZFO and PZT totaling n+1 layers ofCZFO and n layers of PZT (n=5 to 29), with a layer thickness of 10 to 20μm. Structural studies were done with x-ray diffraction on multilayers,crushed powder samples, and samples with exposed interface by etchingaway the top layer. There was no evidence of any impurity phase.Magnetic parameters such as the saturation magnetism, anisotropy, and gvalue were in agreement with values expected. Thus the high temperatureprocessing did not result in any impurity phases or degradation of thequality of the magnetic phase.

EXAMPLE 4

This example illustrates the formation of lithium zinc ferrite-leadzirconate titanate (LZFO-PZT) bilayers and multilayers. Both LZFO-PZTbilayers and multilayers were synthesized from thick films prepared bytape casting. The process involved (i) preparation of submicron-sizedpowder of lithium zinc ferrite (LZFO) and lead zirconate titanate (PZT),(ii) thick-film tapes by doctor-blade techniques, and (iii) laminationand sintering of bilayers and multilayers.

LZFO powder was obtained by standard ceramic techniques and commercialPZT (Sample No. APC850) was obtained from American Piezo Ceramics, Inc.,Mackeyville, Pa.) were used. For tape casting, powders of LZFO and PZTwere each mixed with a solvent such as ethyl alcohol and a dispersantsuch as Blown Menhaden fish oil and ball milled for 24 hours, followedby a second ball mill with a plasticizer such as butyl benzyl phthalateand a binder such as polyvinyl butyral for 24 hours to produce a slurryof LZFO and a slurry of PZT. The slurries were each separately cast intotapes of either LZFO or PZT on a carrier film such as silicon-coatedmylar sheets using doctor-blade techniques. That is the slurries werecast into tapes on a carrier film using a tape caster consisting of apair of stationary micrometer-controlled blades and a moveable castingbed. In general, it was possible to obtain 10×20 cm² tapes with athickness within the range of about 10 to 200 μm. The LZFO and PZT tapeswere then arranged to obtain the desired structure, a bilayer consistingof an LZFO and a PZT tape or multilayers consisting of alternatinglayers of LZFO and PZT tapes. The bilayer or multilayer structures werelaminated under high pressure between about 210 Kgf/cm² and 352 Kgf/cm²(3000 and 5000 psi) and under high temperature of about 400° K. Thelaminated bilayer or multilayer structures were then heated at about1000° K to evaporate the binder. The final sintering of the laminatedbilayer or multilayer was then carried out at between about 1400 and1500° K.

Bilayers were made with 200-μm-thick films of LZFO and PZT. Multilayersconsisted of alternate layers of LZFO and PZT totaling n+1 layers ofLZFO and n layers of PZT (n=5 to 29), with a layer thickness of 10 to 20μm. Structural studies were done with x-ray diffraction on multilayers,crushed powder samples, and samples with exposed interface by etchingaway the top layer. There was no evidence of any impurity phase.Magnetic parameters such as the saturation magnetism, anisotropy, and gvalue were in agreement with values expected. Thus the high temperatureprocessing did not result in any impurity phases or degradation of thequality of the magnetic phase.

EXAMPLE 5

This examples provide data on the effect of Zn-substitution in theferrite phase on magnetoelectric voltage coefficient in ferrite-PZTmultilayers.

NZFO-PZT multilayers were made according to the method shown in Example2, CZFO-PZT multilayers were made according to the method shown inExample 3, and LZFO-PZT multilayers were made according to the methodshown in Example 4.

FIG. 11A shows the transverse magnetoelectric (ME) voltage coefficientverses static magnetic field (Oe) for nickel zinc ferrite-PZT (NZFO-PZT)multilayers for zinc concentrations of x=0.0 (filled circles), x=0.1(triangles), and x=0.2 (diamonds).

FIG. 11B shows the variation of the peak voltage coefficient with x forthe NZFO-PZT multilayers of FIG. 11A.

FIG. 12 shows parallel (λ₁₁) and perpendicular (λ₁₃) magnetostrictionverses static magnetic field (Oe) for NZFO-PZT multilayers with x=0.0(filled circles), x=0.2 (open circles), and x=0.5 (triangles).

FIG. 13 shows the effect of volume ratio (R) wherein R=volume offerrite/volume of PZT on the transverse voltage coefficient in nickelferrite (x=0.1)-PZT multilayers with R=3.0 (diamonds), R=1.8(triangles), and R=0.4 (circles).

FIG. 14 shows parallel magnetostriction verses volume ratio (R) forNZFO(x=0.1)-PZT multilayers with R=0.3 (filled inverted triangles),R=1.0 (filled circles), R=3.0 (open circles), and bulk (open triangles).

FIG. 15A shows the effect of ferrite layers (n) on the magnetoelectric(ME) voltage coefficient in NZFO(x=0.3)-PZT multilayers. The samplescontained n bimetal ferrite and n−1 PZT layers with n=5 (filled invertedtriangles), n=15 (filled circles), n=25 (open circles), and n=30 (opentriangles).

FIG. 15B shows the transverse (filled inverted triangles) andlongitudinal (filled circles) ME voltage coefficients forNZFO(x=0.3)-PZT multilayers with n bimetal ferrite and n−1 PZT layers.

FIG. 16A shows parallel magnetostriction verses static magnetic field(Oe) NZFO(x=0.3)-PZT multilayers wherein n is the number of NZFO layersand n−1 is the number of PZT layers. The samples contained n ferrite andn−1 PZT layers with n=5 (circles), n=15 (squares), and n=30 (diamonds).

FIG. 16B shows perpendicular magnetostriction verses static magneticfield (Oe) for NZFO(x=0.3)-PZT multilayers wherein n is the number ofNZFO layers and n−1 is the number of PZT layers. The samples contained nferrite and n−1 PZT layers with n=5 (circles), n=15 (squares), and n=30(diamonds).

EXAMPLE 6

This example details two primary accomplishments of the nickel ferrite(NFO)-PZT composites: (i) observation of a record high ME coefficientα_(E), 460–1500 mV/cm Oe, in NFO-PZT bilayers and multilayers; and, (ii)a theoretical analysis that accounted very well for the volume andapplied static magnetic-field dependence of α_(E).

Samples of NFO-PZT were synthesized by doctor-blade techniques.Magnetoelectric characterization involved measurements of transverse andlongitudinal α_(E) as a function of bias magnetic field, frequency,temperature and volume fraction for the PE and MS-phases. The NFO-PZTsamples had a transverse effect that was at least an order of magnitudelarger than the longitudinal effect. The ME coefficient was maximum atroom temperature. With increasing frequency, α_(E) was found toincrease. An exponential increase in α_(E) occurred for increasingvolume of the magnetostrictive phase in the multilayer.

The measured α_(E)-values in NFO-PZT were the largest ever measured forany system. For comparison, the best α_(E) value for a single phasematerial was 20 mV/cm Oe for Cr₂0₃ (Astrov, Soviet Phys. JETP 13: 729(1961); Rado and Folen, Phys. Rev. Lett. 7: 310 (1961); Foner andHanabusa, J. Appl. Phys. 34: 1246 (1963)) and was 75 mV/cm Oe formultilayers of CoFe₂O₄ (CFO)-PZT (Harshe et al., Int. J. Appl.Electromagn. Mater. 4: 145 (1993); Avellaneda and Harshe, J. Intell.Mater. Syst. Struct. 5: 501 (1994)).

The main reasons for the high ME effects were (i) the choice of nickelferrite that had a high pseudo-piezomagnetic effect and (ii) theexistence of an ideal NFO-PZT interface as evident from the remarkableagreement between theory and data. In view of the fact that CFO-PZTcomposites had comparable material parameters, the interface played animportant role in the dynamics of ferro-magnetic domain motion and theconsequent stress mediated electromagnetic coupling. Anticipated impactsof the results herein are (i) studies directed at an understanding ofthe physics of the NFO-PZT interface, and (ii) interest in thecomposites for useful technologies.

The materials produced herein are useful for magnetoelectric memorydevices, electrically controlled magnetic devices, magneticallycontrolled piezoelectric devices, and smart sensors (Wood and Austin, inProceedings of the Symposium on Magnetoelectric Interaction Phenomena inCrystals, Seattle, 1973, edited by Freeman and Schmid (Gordon andBreach, New York, 1975), p. 181). In the following sections, details onthe synthesis and magnetoelectric characterization of NFO-PZT bilayersand multilayers are provided. Theoretical estimates based on a modelthat assumes perfect interface bonding have been obtained for comparisonwith data.

I. SAMPLE PREPARATION

Both bilayers and multilayers of NFO-PZT were synthesized from thickfilms prepared by tape casting. The process involves (i) preparation ofsubmicron-size powder of NFO and PZT, (ii) thick-film tapes bydoctor-blade techniques, and (iii) lamination and sintering of bilayersand multilayers. Nickel ferrite powder obtained by standard ceramictechniques and commercial PZT (Sample No. APC850 purchased from AmericanPiezo Ceramics, Inc., Mackeyville, Pa.) were used. For tape casting(Mistler and Twiname, Tape Casting: Theory and Practice (AmericanCeramics Society, Westerville, Ohio, 2000), powders of NFO or PZT weremixed with a solvent (ethyl alcohol) and a dispersant (Blown Menhadenfish oil) and ball milled for 24 hours, followed by a second ballmilling with a plasticizer (butyl benzyl phthalate) and a binder(polyvinyl butyral) for 24 hours. The slurries thus obtained were castinto tapes on silicon-coated mylar sheets using a tape caster consistingof a pair of stationary micrometer-controlled blades and a moveablecasting bed. It was possible to obtain 10×20 cm² tapes with thethickness in the range 10–200 μm. The tapes were arranged to obtain thedesired structure, laminated under high pressure (3000–5000 psi) andhigh temperature (400 K), and heated at 1000 K for binder evaporation.The final sintering was carried out at 1400–1500 K.

Bilayers were made with 200-μm-thick films of NFO and PZT. Multilayerscontained alternate layers of NFO and PZT, totaling n+1 layers of NFOand n layers of PZT (n=5–29), with a layer thickness of 10–20 μm.Structural studies were done with x-ray diffraction on multilayers,crushed power samples, and samples with exposed interface by etchingaway the top layer. There was no evidence for any impurity phases.Magnetic parameters such as the saturation magnetization, anisotropy,and g value were in agreement with values expected for nickel ferrite(Landolt-Börnstein: Numerical Data and Functional Relationships inScience and Technology, edited by Hellwege and Springer,Landolt-Börnstein, New Series, Group III, Vol. 4, Pt. 6(Springer-Verlag, New York, 1970). Thus the high temperature processingdid not result in any impurity phases or degradation of the quality ofthe magnetic phase.

The magnetostriction λ is an important parameter for theoreticalestimates of α_(E) for the composite. In the ferromagnetic phase, themagnetostriction due an ac field δH in the presence of a bias field Hleads to pseudo-piezomagnetic effects that in turn give rise to thenecessary coupling to the piezoelectric phase in the composite. In orderto maximize the ME coefficient, H must correspond to the maximum in theslope of λ vs H characteristics. The standard strain-gauge method(Micro-Measurement Group Strain Indicator-Model 3800 and series WKstrain gauges) and an electromagnet with a maximum field of 5 kOe wereused for the measurement of λ. FIG. 1 shows the static field Hdependence of parallel (λ₁₁) and perpendicular (λ₁₃) magnetostriction,corresponding to H parallel or perpendicular to the sample plane. (Athree-dimensional coordinate system (1,2,3) with the sample in the (1,2)plane was assumed). The room temperature data were for a multilayer withn=14 and a layer thickness of 14 μm. The parallel magnetostriction λ₁₁is negative (the Figure shows the magnitude) and was a factor of 3–10larger than λ₁₃. One observes saturation of λ for H values above 1200Oe. No dependence of λ either on the number of layers or on the layerthickness was observed.

Samples were then polished, electrical contacts were made with silverpaint, and poled. The poling procedure involved heating the sample to420 K and the application of an electric field E of 20 kV/cm. As thesample was cooled to 300 K, E was increased progressively to 50 kV/cmover a duration of 30 min. The dielectric constant for the composite wasin agreement with the expected values for PZT (Piezoelectric CeramicsMaterials Properties (American Piezo Ceramics, Inc., Mackeyville, Pa.,1998). The piezoelectric coefficient, another important parameter forthe piezoelectric phase and the composite, measured with a(Pennebaker-Model 8000) d₃₃ tester was in the range 70–170 pm/V.

II. MAGNETOELECTRIC EFFECTS

The parameter of importance for the multilayers is the magnetoelectricvoltage coefficient α_(E). Magnetoelectric measurements are usuallyperformed under two distinctly different conditions: (i) the inducedmagnetization is measured for an applied electric effect or (ii) theinduced polarization is obtained for an applied magnetic field. Theelectric field produced by an alternating magnetic field applied to thecomposite was measured. A set up in which the sample was subjected to abias field H (with the use of an electromagnet) and an ac field δH (1 Oeat 10 Hz–1 kHz) produced by a pair of Helmholtz coils was used. Thesample was shielded from stray electric fields. Lock-in detection wasimplemented for accurate determination of α_(E) for two different fieldorientations: (i) transverse α_(E) or α_(E3,1) for H and δH parallel toeach other and to sample plane, and δE measured perpendicular to thesample plane and (ii) longitudinal α_(E3,3) for all the three fieldsparallel to each other and perpendicular to the sample plane. Aliquid-helium glass Dewar and a nonmetallic sample insert were used forstudies on temperature dependence of ME effects.

FIG. 18 shows the static magnetic-field dependence of the transverse MEcoefficient α_(E,31), for a two-layer composite of NFO-PZT, each layerwith a thickness of 200 μm. The data at room temperature were for afrequency of 1 kHz and for unit thickness of the piezoelectric phase. AsH was increased from zero, α_(E,31) increases, reached a maximum valueof 460 mV/cm Oe at 70 Oe, and then dropped rapidly to zero above 300 Oe.There was no evidence for hysteresis in FIG. 18 except at fields closeto zero. A phase difference of 180° between the induced voltages for +Hand −H was observed. As discussed later, the magnitude and the fielddependence in FIG. 18 are related to the slope of λ vs H characteristicsin FIG. 17 and can be understood in terms of pseudo-piezomagneticeffects in nickel ferrite.

Similar field dependence of both the transverse and longitudinal MEvoltage coefficients, α_(E,31) and α_(E,33) respectively, are shown inFIG. 19 for a multilayer composite. The data at room temperature for a 1kHz ac field were for a sample consisting of alternate layers of NFO andPZT, each layer with a thickness of 14 μm so that the effectivethickness of PZT was the same as for the bilayer.

A comparison of data in FIGS. 18 and 19 indicated the following. Inmultilayers, (i) ME effects were observed over a wider field range, (ii)the field for maximum α_(E,31) was shifted to higher fields, and (iii)the peak value of α_(E,31) was 15% smaller than for the bilayer. FIG. 19shows a noticeable hysteresis in the field dependence of α_(E,31) andα_(E,33). The variation of α_(E,33) with H was linear up to 1000 Oe, andthe longitudinal ME effect was almost an order of magnitude weaker thanthe transverse effect.

The variation of the multilayer α_(E,31) with frequency and temperatureare shown in FIGS. 20A and 20B. Upon increasing the frequency from 20 Hzto 10 kHz, there was an overall increase of 25% in the ME voltagecoefficient, but a substantial fraction of the increase occurred overthe 1–10 kHz range (except at 353 K). These variations were most likelydue to frequency dependence of the dielectric constant for theconstituent phases and the piezoelectric coefficient for PZT.

Data on temperature dependence of α_(E,31) at 100 Hz are shown in FIGS.20A and 20B. A peak in α_(E,31) was observed at room temperature and itdecreased when T was either increased or decreased from roomtemperature. Detailed temperature dependence of material parameters forboth phases was necessary for an understanding of these results.

According to a theoretical model to be discussed in the followingsection, α_(E) was expected to be dependent sensitively on the ratio ofvolumes of the magnetostrictive (m) and piezoelectric (p) phases,f=v_(m)/v_(p). Data was obtained on such dependence in multilayers ofnickel ferrite (with a small substitution of Zinc for Ni)-PZT. Studieswere performed on multilayers with n=10 and a series of layerthicknesses to obtain samples with the necessary volume fractions. FIG.21 shows the H dependence of α_(E,31) on f values. The observed featureswere similar to the field dependence in FIG. 19. With increasing f, arapid increase in α_(E,31) was evident in FIG. 21. Another observationof importance was the up-shift in the bias magnetic field correspondingto maximum α_(E,31) with increasing f. FIG. 22 shows the variation ofthe peak value of α_(E,31) with f. There was an exponential increase inα_(E,31) as the volume of the magnetostrictive component was increasedand it leveled at high volumes. The data indicated more than an order ofmagnitude increase in α_(E) as f was increased from 0.2 to 5.5 and amaximum of 1500 mV/cm Oe was obtained for f=2.2.

The most significant results in FIGS. 18 to 22 were that (i) for equalvolume of MS and FE phases, the maximum α_(E,31) ranged from 400 mV/cmOe in multilayers to 460 mV/cm Oe in the bilayer and (ii) α_(E,31)increased with increasing volume of nickel ferrite and the largestmeasured value was 1500 mV/cm Oe for v_(m)/v_(p)=2.2. These values mustbe compared with 20 mV/cm Oe for Cr₂0₃, the prior art best single-phaseME material (Astrov, Soviet Phys. JETP 13: 729 (1961); Rado and Folen,Phys. Rev. Lett. 7: 310 (1961); Foner and Hanabusa, J. Appl. Phys. 34:1246 (1963)). The values herein were more than an order of magnitudehigher than reported values for ferrite-BaTiO₃ bulk composites (Van denBoomgaard et al., J. Mater. Sci. 9: 1705 (1974); Van den Boomgaard etal., Ferroelectrics 14: 727 (1976); Van den Boomgaard and Born, J.Mater. Sci. 13: 1538 (1978)) and multilayers of CFO-PZT (Harshe et al.,Int. J. Appl. Electromagn. Mater. 4: 145 (1993); Avellaneda and Harshe,J. Intell. Mater. Syst. Struct. 5: 501 (1994); Harshe, Ph.D. thesis,Pennsylvania State University, 1991) and a factor of 5 larger than thatin laminated composites of Ni(Co,Cu)—Mn ferrite-PZT (Lupeiko et al.,Inorg. Mater. (Transl. of Neorg. Mater.) 31: 1245 (1995)).

III. DISCUSSION

First a theoretical model for the bilayers and estimate the biasmagnetic field and volume-fraction dependence of the magnetoelectricvoltage coefficients for comparison with data is discussed. Followingthis, the possible cause of the large ME effect in NFO-PZT structures isdiscussed. Harshe and co-workers provided a model for a two-layerstructure in which no electric field is present in the MS layer, the topand bottom surfaces of the piezoelectric (PE) layers are equipotentialsurfaces, and the bias magnetic field H is uniform throughout the sample(Harshe et al., Int. J. Appl. Electromagn. Mater. 4: 145 (1993);Avellaneda and Harshe, J. Intell. Mater. Syst. Struct. 5: 501 (1994)).For the composite of interest, NFO-PZT, the assumptions are justifiedbecause of low electrical resistivity for NFO compared to PZT. The totalstrain at the interface is given by the expressionS=sT+dE+qH.  (3)The three terms correspond to contributions from elastic (s compliancecoefficient; T, stress tensor), piezoelectric (d, piezoelectriccoefficient), and pseudopiezomagnetic effects (q, piezomagneticcoefficient). Although strain due to magnetostriction is λH², underappropriate bias field a linear pseudopiezomagnetic is expectedresulting in a strain qH. The ME coefficient α_(E) is estimated by thefollowing procedure: (i) for an applied magnetic field δH, the totalstrain at the interface, which is the sum of elastic andmagnetostrictive strains, is estimated; (ii) the strain-stressrelationship is calculated for boundary conditions at the interface;and, (iii) the stress-electric field (δE) relationship is estimated forappropriate boundary conditions. We are interested in a free body with aperfectly bonded interface. Denoting the magnetostrictive and thepiezoelectric phases by m and p, respectively, one obtains for thetransverse ME coefficientα_(E,31) =δE ₃ /δH ₁=−2d ^(p) ₃₁ q ^(m) ₁₁ v _(m)(s ^(m) ₁₁ +s ^(m)₁₂)ε^(T,p) ₃₃ v _(p)+(s ^(p) ₁₁ +s ^(p) ₁₂)ε^(T,p) ₃₃ v _(m)−2(d ^(p)₃₁)² v _(m)  (5)and the longitudinal coefficientα_(E,33) =δE ₃ /δH ₃=−2d ^(p) ₃₁ q ^(m) ₁₁ v _(m)(s ^(m) ₁₁ +s ^(m)₁₂)ε^(T,p) ₃₃ v _(p)+(s ^(p) ₁₁ +s ^(p) ₁₂)ε^(T,p) ₃₃ v _(m)−2(d ^(p)₃₁)² v _(m)  (6)Here u denotes the volume and ε^(T) is the permittivity at constantstress. Since the ME voltage is induced in the PE phase, Equations (4)and (5) are expressions for per unit thickness of the PE phase. It isclear from Equations (4) and (5) that α_(E) is directly proportional tothe product of piezomagnetic and piezoelectric coefficients, and isdependent on the volume fraction v_(m)/v_(p).

The above model was used for the theoretical calculation of α_(E) forcomparison with the data. The following material parameters were usedfor the constituent phases (Harshe, Ph.D. thesis, Pennsylvania StateUniversity, 1991; Landolt-Börnstein: Numerical Data and FunctionalRelationships in Science and Technology, edited by Hellwege andSpringer, Landolt-Börnstein, New Series, Group III, Vol. 4, Pt. 6(Springer-Verlag, New York, 1970; Piezoelectric Ceramics MaterialsProperties (American Piezo Ceramics, Inc., Mackeyville, Pa., 1998).

PZ, s^(p) ₁₁ = 14 × 10⁻¹² m²/N NiFe₂O₄, s^(m) ₁₁ = 6.5 × 10⁻¹² m²/Ns^(p) ₁₁ = −8 × 10⁻¹² m²/N s^(m) ₁₂ = −2.4 × 10⁻¹² m²/N ε^(T, p) ₃₃ = 17× 10⁻⁹ F/mMeasured values of the piezoelectric coupling coefficient δhd 33 for themultilayers ranged from 70–170 pm/V at 100 Hz and all average value ofd₃₁=d₃₃/2=60 pm/V was used. The other required parameter, q=δλ/δH wasdetermined from parallel (λ₁₁) and perpendicular (λ₁₃) magnetostrictiondata in FIG. 17. Theoretical values (The SI unit for ME voltagecoefficient, (V/m)/A/m)+0.8 V/cm Oe) of α_(E,33) and α_(E,31) wereestimated from Eqs. (4) and (5).

In FIGS. 23A and 23B, calculated transverse and longitudinal ME voltagecoefficients for v_(m)/v_(p)=1 were compared with the data for thebilayer and multilayer (in FIGS. 18 and 19). There was overall agreementfor both the magnitude and field dependence. Recall that α_(E) isfrequency dependent and the agreement between theory and data improvesat high frequencies since α_(E) values are 15% higher at 10 kHz than at1 kHz. The H dependence of α_(E) essentially tracked the slope of λ vsH. Once the magnetostriction attained the saturation value, the loss ofpseudopiezomagnetic coupling led to the absence of ME effects. It isclear from FIGS. 23A and 23B that for the bilayer there was goodagreement between low-field data and theory. However, it was necessaryto explore the reason for the rapid drop in α_(E,31) at high fields. Forthe multilayer, (i) the expected order of magnitude difference inα_(E,31) and α_(E,33) agreed with the data, (ii) the estimated H valuecorresponding to maximum in α_(E,31) was smaller than observedexperimentally, and (iii) there was exceptionally good agreement betweentheory and data for the longitudinal coefficient α_(E,33).

Theoretical values of the transverse ME coefficient are shown in FIG. 22as a function of the volume of the constituent phases and there was goodagreement with the data. Increase in the volume of the MS phase leads toan enhancement in the strain due to magnetostriction and an increase inthe ME coefficient. Our recent theoretical modeling of the reciprocaleffect, i.e., shift in the ferromagnetic resonance field of the MS phasedue to piezoelectric effects in the PE-phase, indicates increasingresonance-field shift with increasing volume of the PE phase (Bichurinet al., Phys. Rev. B 64: 094409 (2001)). Thus, although the theory isbased on a simple two-layer structure, it accounts very well for boththe magnitude and volume and H dependence.

Next to be addressed was what caused the large ME effects in NFO-PZT.The large α_(E,31) was in part due to inherent advantages in MLgeometry: efficient poling and the total absence of leakage currents.But other systems such as CFO-PZT also have the same advantages. Inferrites, domains are spontaneously deformed in the magnetizationdirection and the Joule magnetostriction is caused by domain-wall motionand domain rotation in the presence of H. Since ME effects involvedynamic magnetoelastic coupling, key requirements for the ferrite phaseare unimpeded domain motion and a large λ. A soft, high initialpermeability (low coercivity), and high-λ ferrite, such as NFO usedhere, was the main ingredient for strong ME effects. In magneticallyhard cobalt ferrite, however, there is the disadvantage of a largecoercive field that limits domain rotation. Since the ME effectoriginates at the interface, it is important to consider the influenceof growth-induced stress and its effect on magnetic anisotropy and thedynamics of domain motion. The interface coupling is also influenced bya variety of other factors such as defects, inhomogeneities, and grainboundaries that pin the domain and limit wall motion and rotation.Important factors that affect the interface properties during thehigh-temperature processing are the thermal-expansion coefficients (2ppm for PZT vs 10 ppm for most ferrites), thermal conductivity (an orderof magnitude higher in CFO compared to PZT or NFO) (Landolt-Börnstein:Numerical Data and Functional Relationships in Science and Technology,edited by Hellwege and Springer, Landolt-Börnstein, New Series, GroupIII, Vol. 4, Pt. 6 (Springer-Verlag, New York, 1970; PiezoelectricCeramics Materials Properties (American Piezo Ceramics, Inc.,Mackeyville, Pa., 1998)), and the sintering temperature. Differentialthermal expansion and thermal conductivity could result in built-instrain and interface micro-cracks. The sintering temperature of1425–1500 K is much closer to the melting temperature of cobalt ferrite(1840 K) than NFO (2020 K) and it is quite likely that the highlyreactive lead compound (PZT) could easily form both structural andchemical inhomogeneities at the interface with cobalt ferrite(Landolt-Börnstein: Numerical Data and Functional Relationships inScience and Technology, edited by Hellwege and Springer,Landolt-Börnstein, New Series, Group III, Vol. 4, Pt. 6(Springer-Verlag, New York, 1970). Studies show extensive microcracksand loss of Fe from the ferrite in bulk composites of CFO—BaTiO₃ (Vanden Boomgaard et al., J. Mater. Sci. 9: 1705 (194); Van den Boomgaard etal., Ferroelectrics 14: 727 (1976)). So it is reasonable to concludethat the giant ME effect in NFO-PZT is most likely due to an interfacefree of growth-induced stress or defects and a favorable domaindynamics. Investigations on the microscopic nature of the NFO-PZTinterface with techniques such as high-resolution X-ray diffraction,electron microscopy, and magnetic-force microcopy are criticallyimportant for an understanding of the current observations.

IV. CONCLUSION

Thick-film bilayers and multilayers of nickel ferrite-PZT prepared bytape-casting techniques showed the strongest magnetoelectric effectsreported to date in any system. The effect was of the same strength inbilayers and multilayers for the same effective thickness of theconstituent phases. A general increase in α_(E) was observed withincreasing volume of the magnetostrictive phase. A theoretical modelthat assumed ideal interface conditions accounted very well for themagnitude, and volume and field dependence of ME parameters and;therefore, implied a perfectly bonded defect-free interface. Thefield-conversion efficiency for the composite was well within the rangeneeded for practical applications.

While the present invention is described herein with reference toillustrated embodiments, it should be understood that the invention isnot limited hereto. Those having ordinary skill in the art and access tothe teachings herein will recognize additional modifications andembodiments within the scope thereof. Therefore, the present inventionis limited only by the claims attached herein.

1. In a method for forming a magnetoelectric composite wherein acombination of a piezoelectric composition and a magnetostrictivecomposition are joined together as a laminate in alternate separatelayers to provide a bilayers or multilayers, the improvement whichcomprises: (a) providing a first layer of a bimetal ferrite powder mixedwith a first binder with a thickness between 10 and 200 μm, wherein zincis one of the metals, as the magnetostrictive composition; (b) forming acombination of the first layer of step (a) with a second layer of apowder of a piezoelectric material mixed with a second binder with athickness between 10 to 200 μm in the separate layers; and (c)compressing and sintering the combination of step (b) to form themagnetoelectric composite, wherein the compression of the first andsecond layer is between about 3000 and 5000 psi to provide greenlaminated bilayers or multilayers which are heated to 1050 to 1200° C.to evaporate the binder and then finally sintered at about 400 to 1500°K, wherein the piezoelectric material of the first layer of composite iselectrically poled and the composite has a magnetoelectric voltagecoefficient of at least 100 mV/cm Oe measured at a frequency of 100 Hzat room temperature.
 2. The method of claim 1 wherein the piezoelectricmaterial is selected from the group consisting of lead zirconatetitanate (PZT), lead zincate niobate (PZN), lead zincate niobatelead-titanate (PZN-PT), lead magnesium niobate lead-titanate (PMN-PT),lead lanthanum zirconate titanate (PLZT), Nb/Ta doped-PLZT, and bariumzirconate titanate (BZT).
 3. The method of claims 1 or 2 wherein thebimetal ferrite has the formula:Co_(1−x)Zn_(x)Fe₂O₄ where x is 0.2 to 0.5.
 4. The method of claims 1 or2 wherein the bimetal ferrite has the formulaNi_(1−x)Zn_(x)Fe₂O₄ where x is 0.1 to 0.5.
 5. The method of claims 1 or2 wherein the bimetal ferrite has the formula:Li_(0.5−x/2)Zn_(x)Fe_(2.5−x/2)O₄ where x is 0.1 to 0.4.